Light sensitive invisible ink compositions and methods for using the same

ABSTRACT

A high-reliability invisible ink printing system is provided which generates high-definition, lightfast images that are easily read or otherwise detected using far red, infrared, and/or ultraviolet light. The inks contain an uncomplexed invisible metal phthalocyanine far red/infrared fluorophore (optimally chloroaluminum [III] phthalocyanine tetrasulfonic acid or salts thereof). An ultraviolet fluorophore can also be included. The inks are invisible to the unaided eye. However, when exposed to far red or infrared light (wavelength=about 650-715 nm) they will fluoresce at a wavelength of about 670-720 nm. If an ultraviolet fluorophore is employed, the inks can also be detected by applying ultraviolet light (wavelength=about 250-380 nm) which results in fluorescent emission at a wavelength of about 400-650 nm. The inks are well suited for delivery using inkjet technology to efficiently form invisible images.

BACKGROUND OF THE INVENTION

The present invention generally relates to ink compositions and deliverysystems associated therewith, and more particularly to invisible inkmaterials which include one or more chemical agents therein that cannotbe seen by the unaided eye but will fluoresce when exposed to (1) farred ("FR") light; (2) infrared ("IR") light; or (2) ultraviolet ("UV")light, depending on the particular chemical agents being used. Theresulting fluorescent emission can then be detected through the use of asuitable detection system when far red/infrared light is applied orobserved by the unaided eye when ultraviolet illumination is employed.The invisible ink compositions have many uses in a variety of differentfields and represent an advance in invisible imaging technology.

In recent years, the demand for effective "invisible" ink compositionshas steadily increased. Invisible ink materials are traditionallydefined to involve a broad class of ink formulations which cannot beseen by the unaided eye when applied to a substrate and viewed with"natural" light (e.g. light from the sun) or light from conventionalincandescent lamps and the like. Both of these light forms (as well asother forms which are normally used for general illumination purposes inhomes, businesses, and the like) are collectively characterized as"white" light which involves a combination of all the various coloredlight fractions which fall within a wavelength range of about 300-700nm. Under these illumination conditions, the ink compositions areessentially colorless. Only after illumination with other, more narrowlight wavelengths do the printed images become visible or otherwisedetectible (either with or without auxiliary observation equipment).

The uses of these materials are varied and widespread. For example,invisible ink products provide many benefits when printed on a varietyof documents including insurance policies, checks, and other relatedmaterials. Of particular interest is the use of invisible inks onpaperwork prepared by financial institutions (e.g. checks, accountstatements, routing documents, and the like). These items typicallycontain a wide variety of routing codes, numerical identifiers, datasummaries, and the like which (for numerous reasons includingsecurity-related issues) preferably remain invisible to the unaided eyeunder the conditions outlined above. Likewise, in many applications, itis desired that "bar-coding" for inventory control, product assemblyapplications in factories, and other comparable purposes be undertakenin a manner where the particular bar-code of interest does not appear invisible form on the products or documents being processed.Representative patents which discuss the use of invisible ink materialsfor bar-coding purposes include but are not limited U.S. Pat. No.5,282,894 to Albert et al.; U.S. Pat. No. 5,348,348 to Hanada et al.;and U.S. Pat. No. 5,686,725 to Maruyama et al. Other patents whichgenerally describe the usefulness of invisible ink materials for avariety of different purposes include U.S. Pat. No. 4,243,694 toMansukhani; U.S. Pat. No. 4,540,595 to Acitelli et al.; U.S. Pat. No.5,093,147 to Andrus et al.; U.S. Pat. No. 5,215,838 to Tam et al.; U.S.Pat. No. 5,301,044 to Wright; U.S. Pat. No. 5,423,432 to Krutak et al.;U.S. Pat. No. 5,614,008 to Krutak et al.; U.S. Pat. No. 5,643,356 toNohr et al.; U.S. Pat. No. 5,684,069 to Auslander; U.S. Pat. No.5,702,511 to de Saint-Romain et al.; U.S. Pat. No. 5,703,229 to Krutaket al., and others.

In addition to the various patents which exist involving invisible inkmaterials in general, a number of patents have been granted whichdescribe specific approaches for handling and formulating invisible inkcompositions. For example, certain references disclose invisible dyecompounds which are "complexed" (e.g. chemically coupled or otherwisejoined) with a variety of polymeric materials (see U.S. Pat. No.5,614,008 cited above). The polymers are apparently designed to increasethe fluorescence intensity of the dyes. Notwithstanding the benefitsassociated with this process, the presence of polymeric materials withinthe completed ink formulations (particularly those that are complexedwith the selected dye compound[s]) can diminish print quality levels andreduce overall printer reliability in applications involving highspeed/high resolution inkjet printing. This situation can occur becausethese polymers often form undesired films or deposits within theprinting system which interfere or otherwise prevent effective ink dropformation. As a result, images with poor print quality and inadequateedge acuity are generated.

With continued reference to inkjet technology, this approach is ofconsiderable interest in the marking of substrates using invisible inks.Inkjet printing techniques are characterized by a high degree ofoperational efficiency, low cost, excellent print quality, and rapid inkdelivery. Thermal inkjet printing units are especially important in thisregard. Printing systems which employ thermal inkjet technologybasically involve a cartridge unit having at least one ink reservoirchamber in fluid communication with a printhead. The printhead includesa substrate (preferably made of silicon) that comprises a plurality ofthin-film heating resistors thereon. Selective activation of theresistors causes thermal excitation of the ink materials retained withinthe ink cartridge and expulsion thereof from the cartridge.Representative thermal inkjet systems are discussed in U.S. Pat. No.4,500,895 to Buck et al.; U.S. Pat. No. 4,771,295 to Baker et al.; U.S.Pat. No. 5,278,584 to Keefe et al; and the Hewlett-Packard Journal, Vol.39, No. 4 (August 1988), all of which are incorporated herein byreference. Further information regarding inkjet printing devices(including those which incorporate thermal inkjet technology) will bediscussed below relative to the present invention.

The invention claimed herein shall be applicable to all types of inkjetprinting systems including those which employ cartridge units having aself-contained supply of ink within a housing that is directly attachedto a printhead, as well as alternative inkjet systems which use an inksupply that is remotely positioned from the printhead and in fluidcommunication therewith using one or more conduit members. The claimedmaterials and methods are also applicable to inkjet printing units usingother (e.g. non-thermal) ink delivery methods including those whichincorporate, for example, piezoelectric technology as discussed furtherbelow.

Inkjet printing techniques and the use of invisible ink materials forthe purposes outlined above (and other related applications) offer manyimportant benefits. In accordance with the specialized componentsemployed in inkjet printing systems (particularly thermal inkjet units)which typically include numerous small openings, passageways, and thelike through which ink materials must pass, the inks selected for use inthese systems must be carefully considered. Otherwise, print qualitydeterioration and a decrease in operating efficiency can occur. Inaddition to these factors, the ink materials of interest must complywith many other requirements including high levels of waterfastness,lightfastness, fluorescence intensity, bleed resistance, and the like.The present invention involves specialized invisible ink compositionswhich are particularly well-suited for use in inkjet printing systems(especially those which employ thermal inkjet technology). Likewise, thematerials and methods described herein overcome numerous problemsassociated with prior invisible ink formulations and offer manyadvantages including but not limited to (1) high print quality levels(particularly when thermal inkjet technology is employed); (2) superiorlightfastness and waterfastness; (3) excellent fluorescence intensityduring illumination with an appropriate light source; and (4) a highlevel of reliability when used in connection with inkjet printingsystems (particularly those which employ thermal inkjet technology).

Accordingly, the present invention represents an advance in the art ofinvisible ink imaging which satisfies a long-felt need as noted above.It will become readily apparent from the following discussion that theinvention is novel in the materials and procedures that it employs, aswell as the results which it obtains. The claimed invention thereforeconstitutes a unique development of considerable significance which willnow be discussed in detail.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel invisibleink composition which contains a highly sensitive fluorophoric compoundthat cannot be seen by the unaided eye but fluoresces with considerableintensity when illuminated with far red ("FR") or infrared ("IR") light(characterized herein as a far red/infrared fluorophore or fluorophoriccompound).

It is another object of the invention to provide a novel invisible inkcomposition which contains a highly sensitive far red/infraredfluorophoric compound that is uncomplexed with any polymeric additivesor other ingredients.

It is another object of the invention to provide a novel invisible inkcomposition which, in an alternative embodiment, likewise contains (incombination with the far red/infrared fluorophore) an ultravioletfluorophoric compound which cannot be seen by the unaided eye butfluoresces with considerable intensity when illuminated with ultraviolet("UV") light.

It is another object of the invention to provide a novel invisible inkcomposition which is highly suitable for use with inkjet printingsystems (especially those that employ thermal inkjet technology).

It is another object of the invention to provide a novel invisible inkcomposition which is waterfast, lightfast, and capable of producing highresolution printed images.

It is another object of the invention to provide a novel invisible inkcomposition in which the fluorophoric materials therein have fluorescentproperties that enable them to be readily observed using a minimalamount of detection equipment.

It is another object of the invention to provide a novel invisible inkcomposition which is capable of being used in a highly reliable mannerwith a wide variety of printing systems.

It is another object of the invention to provide a novel invisible inkcomposition which provides the foregoing benefits while using a minimalnumber of chemical ingredients.

It is a further object of the invention to provide a novel invisible inkcomposition which is able to form clear and distinct printed images on awide variety of substrates, with "special" substrates not beingrequired.

It is a further object of the invention to provide a novel invisible inkcomposition which is suitable for use in many different applications,environments, and situations.

It is a still further object of the invention to provide a novelprinting method which employs the invisible ink materials describedabove.

It is a still further object of the invention to provide a novelprinting method which employs the invisible ink materials describedabove wherein inkjet technology is used to generate printed images.

It is an even further object of the invention to provide a novelprinting method which employs the invisible ink materials describedabove in a thermal inkjet printing apparatus.

It is an even further object of the invention to provide a novelprinting method which employs the invisible ink materials describedabove that is rapid, accurate, and reliable with a high level of printquality.

A brief summary of the invention and its benefits will now be provided,with specific details thereof being recited in the Detailed Descriptionof Preferred Embodiments section. The present invention involves ahighly effective invisible ink composition and image generation methodwhich can be used in many different environments. In particular, thevarious embodiments of the claimed ink composition are able to generateprinted images on a selected substrate (including paper materials) whichcannot be seen by the unaided eye when applied to a substrate and viewedwith "natural" light (e.g. light from the sun) or light fromconventional incandescent lamps and the like. Both of these light forms(as well as other forms which are normally used for general illuminationpurposes in homes, businesses, and the like) are collectivelycharacterized as "white" light which involves a combination of all thevarious colored light fractions which fall within a wavelength range ofabout 300-700 nm. Under these illumination conditions, the inkcompositions are essentially colorless. Only after illumination withother, more narrow light wavelengths do the printed images becomevisible to the observer (either with or without auxiliary observationequipment).

To observe the "invisible" printed images, they must be illuminated witheither far red or infrared light (e.g. light within an optimal andnon-limiting wavelength range of about 650-715 nm which encompasses boththe far red and infrared wavelengths of primary interest) or ultraviolet("UV") light (e.g. light within an optimal and non-limiting wavelengthrange of about 250-380 nm), depending on the particular embodiment underconsideration. Regardless of the specific embodiment selected for use ina given situation, the ink compositions described herein (and printingmethods associated therewith) enable printed images to be generated witha high level of image quality while avoiding the difficultiesexperienced by conventional invisible inks. Polymeric additives or"complexing agents" are not required in the ink formulations discussedbelow, with the claimed dye compounds being characterized as"uncomplexed". The term "uncomplexed" as used herein shall involve asituation in which the dyes of interest are not chemically linked to anyparticular materials (especially polymeric compounds) and do not formany dye "complexes". As a result, the overall ability of the inks tofunction effectively in many different printing systems with highreliability levels is increased. This is particularly true when thermalinkjet systems are employed to generate invisible images in a high-speedmanner with minimal "down-time". Additional benefits and specificinformation regarding the ink formulations and printing methods of theinvention will be presented in the Detailed Description of PreferredEmbodiments section, with the claimed products and processesrepresenting a considerable advance in the art of invisible imagingtechnology.

In a primary embodiment of the invention, an ink composition is providedwhich includes an invisible dye comprising an uncomplexed invisiblemetal phthalocyanine fluorophore of the far red/infrared variety whichis optimally water-soluble. Materials that are "invisible" as discussedherein involve compositions which cannot be seen by the unaided eyeunder the conditions expressed above. The term "fluorophore" generallyinvolves a chemical composition which is capable of absorbing light andthereafter emitting fluorescent light upon excitation with light of agiven wavelength. Phthalocyanines (as a group) are basically defined toinclude four isoindole groups (e.g. [(C₆ H₄)C₂ N]) which are linkedtogether to form a complex conjugated structure. Metal phthalocyaninematerials contain one or more metal atoms therein which arestrategically located in the phthalocyanine structure. The term"uncomplexed" is defined above and encompasses metal phthalocyaninecompounds that are not chemically linked with any other materials(including organic polymers) to form complex molecules as used in priorsystems such as those discussed in U.S. Pat. No. 5,614,008. Of primaryinterest in this case is the use of a novel uncomplexed invisiblealuminum phthalocyanine fluorophore, with further information regardingthis composition being provided below.

As previously noted, the use of dye-polymer complexes can presentreliability and image-quality problems in systems which, for example,employ thermal inkjet technology on a high-speed/high resolution basis(e.g. at least about 600 dpi ["dots-per-inch"] at a frequency of about12-16 kHz or more). The use of an invisible metal (e.g. aluminum)phthalocyanine fluorophoric dye composition that is uncomplexed andemployed in a "free" state in connection with the wavelength rangesspecified herein represents a novel advance in the art of invisible inkimaging, especially in connection with thermal inkjet technology.

While the present invention in its broadest sense shall not berestricted to any specific uncomplexed invisible metal phthalocyaninefar red/infrared fluorophores, it has been discovered that unexpectedlysuperior results (in terms of image quality, waterfastness,lightfastness, reliability, fluorescence intensity, and the like) areachieved through the use of a special water-soluble uncomplexed aluminumphthalocyanine far red/infrared fluorophore. This particular materialshall be designated herein as "chloroaluminum (III) phthalocyaninetetrasulfonic acid" (or salts thereof) which (in the acid form) involvesthe following structural formula: ##STR1## From a nomenclaturestandpoint, the above-listed composition consists of C₃₂ H₁₆ AlClN₈ O₁₂S₄, with the following "long-hand" name being applicable:chloro[29H,31H-phthalocyanine-2,9,17,24-tetrasulphonato(6-)-N29,N30,N32]-aluminate(4-).As shown in the foregoing formula, four (--SO₃ H) groups are provided.To form salts of this compound, the hydrogen ions in one or more of the(--SO₃ H) groups (e.g. 1-4 of the groups) may be replaced with apositive counterion preferably selected from the group consisting oflithium (Li⁺) sodium (Na⁺), potassium (K⁺), rubidium (Rb⁺), calcium(Ca⁺²), magnesium (Mg⁺²), aluminum (Al⁺³), ammonium (NH₄ ⁺), andwater-soluble ammonium compounds such as the methyl, ethyl, and ethoxyderivatives thereof. All of the selected counterions may be the samewhen more than one of the (--SO₃ H) groups is involved or mixtures ofdifferent counterions can be employed. A representative and non-limitingexample of a salt of the above-listed composition (e.g. sodiumchloroaluminum [III] phthalocyanine tetrasulfonate) is provided asfollows: ##STR2## Again, many different salts are possible (along withvarying "saturation" levels associated with the [SO₃ ⁻¹ ] groups shownabove.) Chloroaluminum (III) phthalocyanine tetrasulfonic acid and saltsthereof are commercially available from the Ciba-Geigy Corp. ofCharlotte, N.C. (USA)/Basel Switzerland under the name "TINOLUX BBS" or"tetrabenzo tetraazaporphine". Likewise, in a preferred and non-limitingembodiment, the completed ink composition will contain about 1-200 ppmor about 0.0001-0.02% by weight of the invisible dye material (e.g. theuncomplexed invisible metal far red/infrared phthalocyanine fluorophorewith particular reference to chloroaluminum [III] phthalocyaninetetrasulfonic acid and salts thereof).

The uncomplexed invisible metal phthalocyanine far red/infraredfluorophoric dye compositions discussed above (including chloroaluminum[III] phthalocyanine tetrasulfonic acid/salt materials) cannot be seenwith the unaided eye (e.g. are "invisible") as defined above. However,in accordance with the fluorophoric character thereof, such materialswill fluoresce with a high degree of intensity (discussed below) whenilluminated with far red or infrared light having a wavelengthsufficient to cause such fluorescence (light within an optimal,non-limiting wavelength range of about 650-715 nm which encompasses boththe far red and infrared wavelengths of primary interest). Thisflourescent emission can then be detected and otherwise characterized(observed) using a suitable detection/observation system. Fluorescentemission associated with the foregoing far red/infrared fluorophores(e.g. the specific and general materials listed above) will optimallyinvolve the generation of light within a wavelength range of about670-720 nm). This light is not visible with the unaided eye and can bedetected using suitable detection devices as specified below.

The claimed ink composition will likewise comprise at least one ink"vehicle" which may include a number of different ingredients incombination. In a preferred embodiment, the ink vehicle will comprise(1) water; (2) at least one organic solvent material (which may alsofunction as a "humectant", namely, a moisture-retaining agent); orpreferably mixtures thereof with these compositions being present invaried proportions as further discussed in the Detailed Description ofPreferred Embodiments section. Exemplary and preferred organicsolvents/vehicles suitable for use in the claimed ink compositioninclude but are not limited to 2-pyrrolidone; ethoxylated glycerol;diethylene glycol; tetraethylene glycol; 1,5-pentanediol;1,3-propanediol; N-methyl pyrrolidone; 2-propanol;2-ethyl-2-hydroxymethyl-1,3-propanediol; and mixtures thereof. At thispoint, it should be emphasized that the present invention and itsvarious embodiments shall not be restricted to any particularcompositions, materials, proportions, amounts, and other parametersunless otherwise stated herein. All numerical values and ranges providedin this description are recited for example purposes only and shallconstitute preferred embodiments of the invention designed to achievemaximum operational efficiency. As a final note regarding the inkcompositions of interest, they may include a number of supplemental(e.g. optional) ingredients outlined in considerable detail belowincluding without limitation surfactants, additional humectants (definedabove), biocides, buffering agents, and the like. Specific and detailedexamples of preferred ink formulations will again be presented in thefollowing Detailed Description of Preferred Embodiments section.

In a second embodiment, an alternative ink composition is provided whichincludes all of the ingredients listed in connection with the firstembodiment (including the uncomplexed invisible metal phthalocyanine farred/infrared fluorophore as a general class of materials and thepreferred composition recited above [chloroaluminum (III) phthalocyaninetetrasulfonic acid or salts thereof]). Accordingly, the previousdiscussion involving the first ink composition shall be incorporated byreference relative to the second ink composition now being described.The main difference between both ink formulations involves the additionof a second invisible dye composition to the alternative ink product,with the second dye comprising at least one invisible ultravioletfluorophore which cannot be seen by the unaided eye in "white" light orother comparable light forms as discussed above. However, whenultraviolet light is applied (e.g. light within an optimum, non-limitingwavelength range of about 250-380 nm), the ultraviolet fluorophore willfluoresce in a visible manner (e.g. within an optimum, non-limitingwavelength range of about 400-650 nm) and is thereby observable with theunaided eye. Incidentally, in this embodiment, the invisible ultravioletfluorophore discussed above shall be designated herein as a "secondinvisible dye", with the uncomplexed invisible metal phthalocyanine farred/infrared fluorophore being characterized as a "first invisible dye".

This embodiment shall not be restricted to any particular quantities inconnection with both of the above-listed fluorophores (which may bedetermined in accordance with routine preliminary pilot testing).However, optimum results are achieved if the ink composition containsabout 1-200 ppm or about 0.0001-0.02% by weight total combined firstinvisible dye (e.g. the uncomplexed invisible metal phthalocyanine farred/infrared fluorophore as a general class of materials and thepreferred composition recited above [chloroaluminum (III) phthalocyaninetetrasulfonic acid or salts thereof]) and about 500-50000 ppm or about0.05-5% by weight total combined second invisible dye, namely, theclaimed ultraviolet fluorophore[s]. While a number of differentultraviolet fluorophores may be used in connection with the secondinvisible dye without limitation, exemplary and preferred materialssuitable for this purpose include but are not limited to ultravioletabsorbing stilbenes, pyrazolines, coumarins, carbostyrils, pyrenes, andmixtures thereof. Representative materials in each of these classes areas follows: (1) stilbenes:4,4'-bis(triazin-2-ylamino)stilbene-2,2'-disulfonic acid;benzenesulfonicacid-2,2'-(1,2-ethenediyl)bis[5-[4-bis(2-hydroxyethyl)amino]-6-[(4-sulfophenyl)amino]-1,3,5-triazin-2yl]amino-tetrasodiumsalt; and 4,4'-bis[4-diisopropanolamino-6-(p-sulfoanilino)-s-triazin-2-yl-amine]stilbene-sodium disulfonate; (2) pyrazolines: 1,2-diphenyl-2-pyrazoline;(3) coumarins: 7-diethylamino-4-methylcoumarin;7-hydroxy-4-methylcoumarin; and 3-(2-benzimidazolyl)-7-(diethylamino)coumarin; (4) carbostyrils: 2-hydroxyquinoline; and (5)pyrenes: N-(1-pyrenebutanoyl)cysteic acid. Also of interest as anultraviolet fluorophore is dibenzothiophene-5,5-dioxide, as well as C.I.(Color Index) Fluorescent Brightener 28; C.I. Fluorescent Brightener220; and C.I. Fluorescent Brightener 264, with some or all of these C.I.compositions being comparable or structurally equivalent to the specificmaterials listed above. The foregoing ultraviolet fluorophores andothers are commercially available from numerous sources including butnot limited to the Aldrich Chemical Co. of Milwaukee, Wis. (USA); BayerCorporation of Pittsburgh, Pa. (USA) under the names "BLANKOPHORE" or"PHORWHITE"; Ciba-Geigy Corporation of Greensboro, N.C. (USA)/Basil,Switzerland; Molecular Probes of Eugene, Oreg. (USA); Sandoz Chemicalsof Charlotte, N.C. (USA) under the name "LEUKOPHOR"; and Sigma Co. ofSt. Louis, Mo. (USA). These materials are characterized by their abilityto generate fluorescent light upon ultraviolet illumination as discussedherein which can be seen by the unaided eye.

This alternative embodiment of the claimed ink composition will likewisecomprise a number of additional materials therein, all of which aresubstantially the same as those listed above in connection with thefirst embodiment. For example, at least one ink "vehicle" will beemployed which may include a number of different ingredients incombination. In a preferred embodiment, the ink vehicle will againcomprise (1) water; (2) at least one organic solvent material (which mayalso function as a "humectant", namely, a moisture-retaining agent); orpreferably mixtures thereof, with these compositions being present invaried proportions. Exemplary and preferred organic solvents/vehiclessuitable for use in the claimed second ink composition include but arenot limited to 2-pyrrolidone; ethoxylated glycerol; diethylene glycol;tetraethylene glycol; 1,5-pentanediol; 1,3-propanediol; N-methylpyrrolidone; 2-propanol; 2-ethyl-2-hydroxymethyl-1,3-propanediol; andmixtures thereof. As a final note regarding the ink formulationsassociated with this embodiment, they may again contain a number ofsupplemental (e.g. optional) ingredients outlined in considerable detailbelow including without limitation surfactants, additional humectants,biocides, buffering agents, and the like.

Having described the ink compositions of primary interest in this caseand their main components, preferred printing methods using thespecialized ink products of the invention will now be summarized.Basically, the inks may be delivered using a wide variety of printingsystems without limitation. However, in a preferred embodiment, theclaimed inks are particularly suitable for delivery using inkjetprinting units (especially those which employ thermal inkjettechnology). This suitability is based on the particular ingredientschosen for use in the ink compositions (especially the uncomplexedinvisible metal phthalocyanine far red/infrared fluorophore, namely,chloroaluminum [III] phthalocyanine tetrasulfonic acid or saltsthereof). The following discussion shall therefore focus on the use ofinkjet technology to deliver the claimed ink compositions to a selectedsubstrate with the understanding that the inks described herein may alsobe transferred using other diverse printing techniques ranging fromsilkscreen methods to conventional offset processes.

To produce a printed invisible image using the selected inkformulations, an ink delivery system is initially provided. In apreferred embodiment, the ink delivery system will generally beconfigured in the form of an ink cartridge unit (mounted within asuitable printer) which includes a housing having at least oneink-retaining chamber inside. The ink-retaining chamber will contain asupply of invisible ink therein corresponding to either of the twoembodiments listed above. In this regard, the printing method currentlybeing described is equally applicable to all of the ink compositionsdiscussed herein. These compositions again include (1) the product ofthe first embodiment in which the ink composition contains [A] aninvisible dye comprising an uncompleted invisible metal phthalocyaninefar red/infrared fluorophore (with chloroaluminum [III] phthalocyaninetetrasulfonic acid or salts thereof providing optimum results); and [B]an ink vehicle comprising water and/or at least one organic solvent, and(2) the product of the second embodiment in which the ink compositioncontains [A] a first invisible dye comprising an uncompleted invisiblemetal phthalocyanine far red/infrared fluorophore (with chloroaluminum[III] phthalocyanine tetrasulfonic acid or salts thereof again providingoptimum results); [b] a second invisible dye comprising an ultravioletfluorophore; and [c] an ink vehicle comprising water and/or at least oneorganic solvent. Specific information regarding these ink materials islisted above (including the optimal absorption/emission wavelengthcharacteristics thereof), with this information being incorporated byreference in the present discussion of preferred printing methods. Inparticular, it is important to note that the selected uncomplexedinvisible metal phthalocyanine far red/infrared fluorophore willoptimally absorb light within a wavelength range of about 650-715 nm andemit fluorescent light within a wavelength range of about 670-720 nm.Likewise, in the second embodiment of the claimed ink composition, theultraviolet fluorophore will optimally absorb light within a wavelengthrange of about 250-380 nm and emit light within a wavelength range ofabout 400-650 nm.

The ink delivery system also includes a printhead in fluid communicationwith the ink-retaining chamber and ink materials in the housing, withthe printhead comprising at least one ink ejector for expelling inkon-demand from the ink-retaining chamber. In an exemplary and preferredembodiment involving the use of a thermal inkjet apparatus, theprinthead will include a plurality of resistors and an outer platehaving at least one or more ink ejection openings through the plate.

Next, a substrate is provided which is designed to receive the invisibleink. The present invention shall not be limited to any particularsubstrates, with a wide variety of materials being applicable for thispurpose including substrates made from paper, metal, plastic, and thelike. It is an important attribute of the claimed ink formulations andmethods that "special" substrates (including custom-produced paperproducts) are not required.

To initiate the printing process, the printhead of the ink deliverysystem is activated in order to deliver the chosen invisible inkcomposition from the ink retaining chamber of the housing onto thesubstrate. Activation of the printhead in a thermal inkjet system willinvolve selective energization of the resistors in order to heat the inkand thereby expel it from the ink retaining chamber. Ifnon-thermal-inkjet systems are used to deliver the ink, printheadactivation will be accomplished using the particular ink ejectors underconsideration, with the procedures associated therewith varying fromsystem to system. It should also be understood that the printing processdiscussed above is equally applicable to (A) systems in which the inkjetprinthead is directly attached to the housing in order to form anintegral, self-contained cartridge unit having a supply of ink withinthe housing; and (B) systems in which the housing and ink materialstherein are remotely positioned from the printhead and in fluidcommunication therewith using one or more tubular conduits. In thisregard, any statements which indicate that the printhead is in "fluidcommunication" with or "operatively connected to" the ink retainingchamber and housing shall encompass both of the foregoing variations.

In accordance with the steps described above, a printed image isgenerated on the substrate which is not visible to the unaided eye in"normal" or "white" light as discussed herein, with the image thus beingcharacterized as "invisible". As a result, the printed image is highlyuseful in security-related applications. When detection of the image isdesired (and the ink composition of the first embodiment is employedwhich contains a selected invisible far red/infrared fluorophore), lightis applied having a wavelength sufficient to cause the printed image toemit fluorescent light. In order to achieve optimum results in thisembodiment (which again involves the use of an invisible farred/infrared fluorophore including but not limited to chloroaluminum[III] phthalocyanine tetrasulfonic acid or salts thereof), either farred or infrared light is applied to the image. In a preferredembodiment, light within an optimal, non-limiting wavelength range ofabout 650-715 nm is used which encompasses both the far red and infraredwavelengths of primary interest. Within this range, best results may beachieved using a non-limiting wavelength range of about 660-690 nm. Theapplication of light in this manner will cause the ink composition tofluoresce within an optimal, non-limiting wavelength range of about670-720 (best=about 670-710 nm). A high fluorescence level is achievedusing the uncomplexed invisible metal phthalocyanine far red/infraredfluorophore compositions described herein (particularly chloroaluminum[III] phthalocyanine tetrasulfonic acid/salt materials which provideunexpectedly superior results). The resulting fluorescent emission fromthe printed image (which is not visible to the unaided eye) may then bedetected or otherwise observed using a suitable infrared fluorescencedetecting system to be discussed in greater detail below.

In an alternative embodiment, the invisible printed image can beproduced from an ink composition which has (A) the first invisible dye(e.g. the uncomplexed invisible metal phthalocyanine far red/infraredfluorophore with particular emphasis on chloroaluminum [III]phthalocyanine tetrasulfonic acid and salts thereof); and (B) the secondinvisible dye, namely, an invisible ultraviolet fluorophore. Aspreviously noted, the image generated from this dual-fluorophore inkcomposition will likewise be invisible to the unaided eye when viewedunder "normal" or "white" light as previously indicated. Whenobservation/detection of the image is desired, light of a predeterminedwavelength is applied to the invisible printed image which again has awavelength sufficient to cause the printed image to generate fluorescentlight. The light which may be employed for this purpose includes: (1)either far red or infrared light which, in a preferred embodiment, willinvolve an optimal, non-limiting wavelength range of about 650-715 nmwhich encompasses both the far red and infrared wavelengths of primaryinterest, with best results being achieved at about 660-690 nm; and/or(2) ultraviolet light within a preferred and non-limiting wavelengthrange of about 250-380 nm. The application of light in this manner willcause the ink composition to fluoresce in a highly effective manner.Specifically, if far red or infrared light is applied (e.g. within theforegoing range), fluorescent light will be emitted within an optimal,non-limiting wavelength range of about 670-720 nm (best=about 670-710nm). This emitted light (which is not visible to the unaided eye) maythen be detected or otherwise observed using a suitable infraredfluorescence detecting system to be discussed in greater detail below.

If ultraviolet light is applied (e.g. within the foregoing range),fluorescent light will be emitted within an optimal, non-limitingwavelength range of about 400-650 nm. As a result, the printed image maybe seen with the unaided eye and special observation or detectingequipment is not required. Likewise, the unique nature of this"combined" FR/IR/UV fluorophore system will become readily apparent fromthe specific information provided below.

In a system where far red/infrared light and ultraviolet light areapplied in combination to a printed image containing both of thefluorophores listed above, the results will involve a combination of theeffects described above. Specifically, a "dual emission" situation willexist involving fluorescent light from both fluorophores which can beobserved using either of the previously described techniques. Theselection of any given technique (either an appropriate detecting systemor the unaided eye) will depend on whether the emission being observedis from the far red/infrared fluorophore or the ultraviolet fluorophore.

A decision to employ an ink composition containing an uncomplexedinvisible metal phthalocyanine far red/infrared fluorophore(particularly chloroaluminum [III] phthalocyanine tetrasulfonic acid orsalts thereof) by itself or combined with an ultraviolet fluorophorewill depend on the intended use of the marking system. For example, thecombined FR/IR/UV fluorophore technique provides the following benefits:(1) a high degree of flexibility which enables users of the system toemploy either a far red/infrared or ultraviolet light source with asingle ink composition; and (2) an enhanced level of security byrequiring readability in two different wavelength regions. Regardless ofwhich ink formulation is selected for use, the present inventionrepresents a considerable advance in the art of invisible ink imaging.In particular, the claimed inks and printing methods provide manyimportant benefits compared with previously known techniques including ahigh degree of simplicity, applicability to a wide variety of printingsystems with emphasis on thermal inkjet technology, cost-efficiency,superior print quality/uniformity, excellent stability (namely,lightfastness and waterfastness), and the general ability to producecompletely invisible images which are readily detected on-demand by theapplication of far red, infrared, and/or ultraviolet light thereto.

The summary presented above was designed to offer a brief overview ofthe invention and shall not limit the scope thereof in any manner. Amore detailed, fully-enabling, and comprehensive assessment of theinvention including a discussion of the claimed inks and printingtechniques will now be provided. Accordingly, these and other objects,features, and advantages of the invention shall be set forth below inthe Detailed Description of Preferred Embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective schematic view of a representativethermal inkjet cartridge unit which may be used with the novel inkcompositions and printing methods of the present invention.

FIG. 2 is a schematic, sequential representation of the process stepswhich are employed to produce an invisible printed image on a substrateusing the materials and methods of the invention in a primaryembodiment, followed by the procedures that are used to observe orotherwise detect the image.

FIG. 3 is a schematic, sequential representation of the process stepswhich are employed to produce an invisible printed image on a substrateusing the materials and methods of the invention in a secondaryembodiment, followed by the procedures that are used to observe orotherwise detect the image.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A novel marking system for producing invisible printed images on a widevariety of substrates will now be described in detail. The inkcompositions of interest employ special combinations of ingredientswhich clearly depart from previously-known materials. These ingredientsare particularly appropriate for use in inkjet printing systems,especially those which employ thermal inkjet technology. The uniquecharacter of the claimed ink formulations, the specialized processesused to generate invisible images, and the distinctive manner in whichthe images are viewed will become readily apparent from the followingdiscussion.

To provide a clear and complete understanding of the invention, thefollowing description will be divided into three sections, namely, (1)"A. An Overview of Thermal Inkjet Technology"; (2) "B. The InkCompositions"; and (3) "C. Printing and Detecting Methods."

A. An Overview of Thermal Inkjet Technology

The novel ink compositions and printing methods described herein areagain applicable to a wide variety of printing devices (alsocharacterized as "ink delivery systems") which include (1) a housinghaving an internal compartment or chamber therein; (2) a printheaddirectly or remotely connected to the housing and in fluid communicationwith the chamber; and (3) at least one "ink ejector" associated with theprinthead. As previously noted, the term "ink ejector" is defined toencompass any component, system, or device which selectively ejects orexpels ink on-demand from the printhead. Thermal inkjet cartridges whichuse multiple heating resistors as ink ejectors are preferred for thispurpose. However, the claimed invention shall not be restricted to anyparticular ink ejectors or inkjet printing technologies as indicatedabove. Instead, a wide variety of different ink delivery devices areapplicable including but not limited to piezoelectric drop systems ofthe general type disclosed in U.S. Pat. No. 4,329,698 to Smith, dotmatrix systems of the variety described in U.S. Pat. No. 4,749,291 toKobayashi et al., as well as other comparable and functionallyequivalent systems designed to deliver ink using one or more inkejectors. The specific operating components associated with thesealternative systems (e.g. the piezoelectric elements in the apparatus ofU.S. Pat. No. 4,329,698) shall be encompassed within the term "inkejectors" as previously noted.

To facilitate a complete understanding of the claimed invention as itapplies to thermal inkjet technology (which is the preferred system ofprimary interest and novelty), an overview of this technique will now beprovided. A representative thermal inkjet cartridge unit is illustratedin FIG. 1 at reference number 10. It shall be understood that cartridge10 is presented herein for example purposes and is non-limiting. Inaddition, cartridge 10 is shown in schematic format in FIG. 1, with moredetailed information regarding cartridge 10 and its various featuresbeing provided in U.S. Pat. No. 4,500,895 to Buck et al.; U.S. Pat. No.4,771,295 to Baker et al.; U.S. Pat. No. 5,278,584 to Keefe et al.; andthe Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of whichare incorporated herein by reference.

With continued reference to FIG. 1, the cartridge 10 first includes ahousing 12 which is preferably manufactured from plastic, metal, or acombination of both. The housing 12 further comprises a top wall 16, abottom wall 18, a first side wall 20, and a second side wall 22. In theembodiment of FIG. 1, the top wall 16 and the bottom wall 18 aresubstantially parallel to each other. Likewise, the first side wall 20and the second side wall 22 are also substantially parallel to eachother.

The housing 12 further includes a front wall 24 and a rear wall 26.Surrounded by the front wall 24, rear wall 26, top wall 16, bottom wall18, first side wall 20, and second side wall 22 is an interiorink-retaining compartment or chamber 30 within the housing 12 (shown inphantom lines in FIG. 1) which is designed to retain a supply of an inkcomposition 32 therein (either in liquid [uncontained] form, retainedwithin an absorbent foam-type member [not shown], or stored in abladder-like structure [not shown]). The ink composition 32 will involvethe novel ink formulations of the present invention as discussed indetail below.

The front wall 24 further includes an externally-positioned,outwardly-extending printhead support structure 34 which comprises asubstantially rectangular central cavity 50 therein. The central cavity50 has a bottom wall 52 shown in FIG. 1 with an ink outlet port 54therein. The ink outlet port 54 passes entirely through the housing 12and, as a result, communicates with the chamber 30 inside the housing 12so that ink materials can flow outwardly from the chamber 30 through theink outlet port 54.

Also positioned within the central cavity 50 is a rectangular,upwardly-extending mounting frame 56, the function of which will bediscussed below. As schematically shown in FIG. 1, the mounting frame 56is substantially even (flush) with the front face 60 of the printheadsupport structure 34. The mounting frame 56 specifically includes dual,elongate side walls 62, 64.

With continued reference to FIG. 1, fixedly secured to housing 12 of theink cartridge 10 (e.g. attached to the outwardly-extending printheadsupport structure 34) is a printhead generally designated in FIG. 1 atreference number 80. For the purposes of this invention and inaccordance with conventional terminology, the printhead 80 actuallycomprises two main components fixedly secured together (with certainsub-components positioned therebetween). The first main component usedto produce the printhead 80 consists of a substrate 82 preferablymanufactured from silicon. Secured to the upper surface 84 of thesubstrate 82 using conventional thin film fabrication techniques is aplurality of individually-energizable thin-film resistors 86 whichfunction as "ink ejectors" and are preferably fabricated from atantalum-aluminum composition known in the art for resistorconstruction. Only a small number of resistors 86 are shown in theschematic representation of FIG. 1, with the resistors 86 beingpresented in enlarged format for the sake of clarity. Also provided onthe upper surface 84 of the substrate 82 using conventionalphotolithographic techniques is a plurality of metallic conductivetraces 90 (e.g. circuit elements) which electrically communicate withthe resistors 86. The conductive traces 90 also communicate withmultiple metallic pad-like contact regions 92 positioned at the ends 94,95 of the substrate 82 on the upper surface 84. The function of allthese components which, in combination, are collectively designatedherein as a resistor assembly 96 will be discussed further below.

Many different materials and design configurations may be used toconstruct the resistor assembly 96, with the present invention not beingrestricted to any particular elements, materials, and components forthis purpose. However, in a preferred, representative, and non-limitingembodiment, the resistor assembly 96 will be approximately 0.5 incheslong, and will likewise contain 300 resistors 86 thus enabling aresolution of approximately 600 dots per inch ("DPI"). The substrate 82containing the resistors 86 thereon will preferably have a width "W"(FIG. 1) which is less than the distance "D" between the side walls 62,64 of the mounting frame 56. As a result, ink flow passageways areformed on both sides of the substrate 82 so that ink flowing from theink outlet port 54 in the central cavity 50 can ultimately come incontact with the resistors 86 as discussed further below. It should alsobe noted that the substrate 82 may include a number of other componentsthereon (not shown) depending on the type of ink cartridge 10 underconsideration.

Securely affixed to the upper surface 84 of the substrate 82 (with anumber of intervening material layers therebetween including a barrierlayer [not shown]) is the second main component of the printhead 80.Specifically, an orifice plate 104 is provided as illustrated in FIG. 1which is used to distribute the selected ink compositions to adesignated print media material or substrate (e.g. paper). In accordancewith the claimed invention, the orifice plate 104 consists of a panelmember 106 (shown schematically in FIG. 1) which is manufactured fromone or more metal compositions (e.g. gold-plated nickel or othercomparable materials). The orifice plate 104/panel member 106 in arepresentative and non-limiting embodiment has an overall thickness ofabout 10-70 microns and is sized to fit over and conform with thesubstrate 82. However, the present invention shall not be restricted toany particular dimensions in connection with the orifice plate 104, withthe invention being prospectively applicable to many different orificeplate units of variable size and shape.

The orifice plate 104 further comprises at least one and preferably aplurality of openings or "orifices" therethrough which are designated atreference number 108. These orifices 108 are shown in enlarged format inFIG. 1. Each orifice 108 in a representative embodiment will have adiameter of about 0.01-0.05 mm. In the completed printhead 80, all ofthe components listed above are assembled so that each of the orifices108 is aligned with at least one of the resistors 86 (e.g. "inkejectors") on the substrate 82. As result, energization of a givenresistor 86 will cause ink expulsion from the desired orifice 108through the orifice plate 104. The claimed invention shall not belimited to any particular size, shape, or dimensional characteristics inconnection with the orifice plate 104 and shall likewise not berestricted to any number or arrangement of orifices 108. In arepresentative embodiment as presented in FIG. 1, the orifices 108 arearranged in two rows 110, 112 on the panel member 106 associated withthe orifice plate 104. If this arrangement of orifices 108 is employed,the resistors 86 on the resistor assembly 96 (e.g. the substrate 82)will also be arranged in two corresponding rows 114, 116 so that therows 114, 116 of resistors 86 are in substantial registry with the rows110, 112 of orifices 108. Further information concerning this type ofmetallic orifice plate system is provided in, for example, U.S. Pat. No.4,500,895 to Buck et al. which is incorporated herein by reference. Itshould also be noted that, while the system illustrated in FIG. 1involves the use of orifice plates produced from metal compositions,alternative printing systems have effectively employed orifice platestructures constructed from non-metallic organic polymer compositions.Further data regarding the use of non-metallic organic orifice platesystems is provided in U.S. Pat. No. 5,278,584 (incorporated byreference).

With continued reference to FIG. 1, a film-type flexible circuit member118 is likewise provided in connection with the cartridge 10 which isdesigned to "wrap around" the outwardly-extending printhead supportstructure 34 in the completed ink cartridge 10. Many different materialsmay be used to produce the circuit member 118, with representative(non-limiting) examples including polytetrafluoroethylene (e.g.Teflon®), polyimide, polymethylmethacrylate, polycarbonate, polyester,polyamide polyethyleneterephthalate, or mixtures thereof. Likewise, arepresentative commercial organic polymer (e.g. polyimide-based)composition which is suitable for constructing the flexible circuitmember 118 is a product sold under the trademark "KAPTON" by the DuPontCorporation of Wilmington, Del. (USA). The flexible circuit member 118is secured to the printhead support structure 34 by adhesive affixationusing conventional adhesive materials (e.g. epoxy resin compositionsknown in the art for this purpose). The flexible circuit member 118enables electrical signals to be delivered and transmitted from theprinter unit (not shown) to the resistors 86 (or other ink ejectors) onthe substrate 82 as discussed below. The film-type flexible circuitmember 118 further includes a top surface 120 and a bottom surface 122(FIG. 1). Formed on the bottom surface 122 of the circuit member 118 andshown in dashed lines in FIG. 1 is a plurality of metallic (e.g.gold-plated copper) circuit traces 124 which are applied to the bottomsurface 122 using known metal deposition and photolithographictechniques. Many different circuit trace patterns may be employed on thebottom surface 122 of the flexible circuit member 118, with the specificpattern depending on the particular type of ink cartridge 10 andprinting system under consideration. Also provided at position 126 onthe top surface 120 of the circuit member 118 is a plurality of metallic(e.g. gold-plated copper) contact pads 130. The contact pads 130communicate with the underlying circuit traces 124 on the bottom surface122 of the circuit member 118 via openings or "vias" (not shown) throughthe circuit member 118. During use of the ink cartridge 10 in a printerunit, the pads 130 come in contact with corresponding printer electrodesin order to transmit electrical control signals from the printer unit tothe contact pads 130 and traces 124 on the circuit member 118 forultimate delivery to the resistor assembly 96. Electrical communicationbetween the resistor assembly 96 and the flexible circuit member 118will again be outlined below.

Positioned within the middle region 132 of the film-type flexiblecircuit member 118 is a window 134 which is sized to receive the orificeplate 104 therein. As shown schematically in FIG. 1, the window 134includes an upper longitudinal edge 136 and a lower longitudinal edge138. Partially positioned within the window 134 at the upper and lowerlongitudinal edges 136, 138 are beam-type leads 140 which, in arepresentative embodiment, are gold-plated copper and constitute theterminal ends (e.g. the ends opposite the contact pads 130) of thecircuit traces 124 positioned on the bottom surface 122 of the flexiblecircuit member 118. The leads 140 are designed for electrical connectionby soldering, thermocompression bonding, and the like to the contactregions 92 on the upper surface 84 of the substrate 82 associated withthe resistor assembly 96. As a result, electrical communication isestablished from the contact pads 130 to the resistor assembly 96 viathe circuit traces 124 on the flexible circuit member 118. Electricalsignals from the printer unit (not shown) can then travel via theconductive traces 90 on the substrate 82 to the resistors 86 so thaton-demand heating (energization) of the resistors 86 can occur.

It is important to emphasize that the present invention shall not berestricted to the specific printhead 80 illustrated in FIG. 1 anddiscussed above, with many other printhead designs also being suitablefor use in accordance with the claimed invention. Likewise, it shouldalso be noted that if a non-metallic organic polymer-type orifice platesystem is desired, the orifice plate 104 and flexible circuit member 118can be manufactured as a single unit as discussed in U.S. Pat. No.5,278,584.

The final step in producing the completed printhead 80 involvesattachment of the orifice plate 104 in position on the underlyingportions of the printhead 80 so that the orifices 108 are in precisealignment with the resistors 86 on the substrate 82. Representativethermal inkjet cartridge units which incorporate all or part of thecomponents listed above are commercially available from theHewlett-Packard Company of Palo Alto, Calif. (USA) under the followingproduct designations/numbers: 51641A, 51645A, 51640C, 51640A, and 51649Awhich are provided as non-limiting examples. These ink cartridges andink cartridge 10 discussed above in connection with FIG. 1 involve a"self-contained" ink delivery system which includes an "on-board" supplyof ink. The claimed invention may likewise be used with other systems(both thermal inkjet and non-thermal-inkjet) which employ a printheadand a supply of ink stored within an ink containment vessel that isremotely spaced from but operatively connected to and in fluidcommunication with the printhead (also known as an "off-axis" system).Fluid communication is accomplished using one or more tubular conduits.An example of such a system is disclosed in co-owned Pending U.S. patentapplication Ser. No. 08/869,446 filed on Jun. 5, 1997 which isincorporated herein by reference. In this regard, the terms "operativelyconnected" and "in fluid communication" as used to define theinterrelationship between the printhead and the ink containment vesselshall be broadly construed to encompass (A) a system in which the inkcontainment vessel is directly attached to and in fluid communicationwith the printhead to form, for example, a single cartridge unit havingan "on-board" ink supply; and (B) an "off-axis" system as previouslydiscussed in which the ink containment vessel is remotely spaced fromthe printhead and not "directly" attached thereto. Having described anexample of a thermal inkjet cartridge 10 suitable for use in connectionwith the present invention, the claimed novel ink compositions and imagegeneration methods will now be discussed.

B. The Ink Compositions

As outlined in this section, novel colorless ink compositions aredisclosed which are capable of producing images that are "invisible".Invisible ink materials are traditionally defined to involve a broadclass of ink formulations which cannot be seen by the unaided eye whenapplied to a substrate and viewed with "natural" light (e.g. light fromthe sun) or light from conventional incandescent lamps and the like.Both of these light forms (as well as other forms which are normallyused for general illumination purposes in homes, businesses, and thelike) are collectively characterized as "white" light which involves acombination of all the various colored light fractions which fall withina wavelength range of about 300-700 nm. Under these illuminationconditions, the ink compositions described below are essentiallycolorless. Only after illumination with other, more narrow lightwavelengths do the printed images become visible or otherwise detectible(either with or without auxiliary observation equipment). Thespecialized ink compositions outlined in this section are characterizedby the use of novel ingredients which collectively provide improvedimage stability, high print quality levels, the production of imageswhich are detected/viewed using illumination systems of minimalcomplexity, and suitability for delivery to many different substrates byhigh-speed inkjet printers which preferably employ thermal inkjettechnology. The two primary embodiments of the claimed ink compositionwill now be discussed in detail.

1. Ink Composition No. 1

This ink formulation comprises a number of ingredients which cooperateto produce the beneficial results discussed above. Specifically, an inkcomposition is provided which includes an invisible dye comprising atleast one or more uncomplexed invisible metal phthalocyanine farred/infrared fluorophores which are optimally water-soluble. The term"invisible" as used herein is discussed above and again involvesmaterials that cannot be seen by the unaided eye. The word "fluorophore"generally involves a chemical composition which is capable of absorbinglight and thereafter emitting fluorescent light upon excitation withlight of a given wavelength. The composition listed above ischaracterized as a far red ("FR")/infrared ("IR") fluorophore because itis able to emit fluorescent light (discussed below) when illuminated bylight in the far red or infrared spectral regions. Far red lightnormally involves a wavelength range of about 650-700 nm, with infraredlight involving a range which exceeds about 700 nm up to about 1000 nm.In accordance with the present invention, the materials described hereinare primarily designed to fluoresce when illuminated with light in anoptimal, non-limiting wavelength range of about 650-715 nm (whichencompasses the far red and infrared wavelengths of primary interest).This step results in fluorescent light emission within an optimal,non-limiting wavelength range of about 670-720 nm as described later inthis section.

Phthalocyanines (as a group) are basically defined to include fourisoindole groups (e.g. [(C₆ H₄)C₂ N]) which are linked together to forma complex conjugated structure. Metal phthalocyanine materials containone or more metal atoms therein which are strategically located in thephthalocyanine structure. In accordance with a preferred embodiment ofthe invention, aluminum phthalocyanine materials provide excellentresults. The term "uncomplexed" as previously defined encompasses metal(e.g. aluminum) phthalocyanine compounds that are not chemically linkedwith any other materials (including organic polymers) to form complexmolecules as used in prior systems such as those discussed in U.S. Pat.No. 5,614,008. The use of invisible dye-polymer complexes can presentreliability and image-quality problems in systems which, for example,employ thermal inkjet technology on a high-speed/high resolution basis(e.g. at least about 600 dpi ["dots-per-inch"] at a frequency of about12-16 kHz or more). The use of an invisible metal (e.g. aluminum)phthalocyanine fluorophoric dye product that is uncomplexed and employedin a "free" state represents a novel advance in the art of invisible inkimaging, especially in connection with thermal inkjet technology.

While the present invention in its broadest sense shall not berestricted to any specific uncomplexed invisible metal phthalocyaninefar red/infrared fluorophores, it has been discovered that unexpectedlysuperior results (in terms of image quality, waterfastness,lightfastness, reliability, fluorescence intensity, and the like) areachieved through the use of a special water-soluble uncomplexedinvisible aluminum phthalocyanine far red/infrared fluorophore. Thisparticular material shall be designated herein as "chloroaluminum (III)phthalocyanine tetrasulfonic acid" (and salts thereof) which (in theacid form) involves the following structural formula: ##STR3## From anomenclature standpoint, the above-listed composition consists of C₃₂H₁₆ AlClN₈ O₁₂ S₄, with the following "long-hand" name being applicable:chloro[29H,31H-phthalocyanine-2,9,17,24-tetrasulphonato(6-)-N29,N30,N32]-aluminate(4-).This material has an approximate molecular weight of about 895.22. Asshown in the foregoing formula, four (--SO₃ H) groups are provided. Toform salts of this compound, the hydrogen ions in one or more of the(--SO₃ H) groups (e.g. 1-4 of the groups) may be replaced with apositive counterion preferably selected from the group consisting oflithium (Li⁺) sodium (Na⁺), potassium (K⁺), rubidium (Rb⁺), calcium(Ca⁺²), magnesium (Mg⁺²), aluminum (Al⁺³), ammonium (NH₄ ⁺), andwater-soluble ammonium compounds such as the methyl, ethyl, and ethoxyderivatives thereof. All of the selected counterions may be the samewhen more than one of the (--SO₃ H) groups is involved or mixtures ofdifferent counterions can be employed. A representative and non-limitingexample of a salt of the above-listed composition (sodium chloroaluminum[III] phthalocyanine tetrasulfonate) is provided as follows: ##STR4##Again, many different salts are possible (along with varying"saturation" levels associated with the [SO₃ ⁻¹ ] groups shown above.)Chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereofare commercially available from the Ciba-Geigy Corp. of Greensboro, N.C.(USA)/Basel Switzerland under the name "TINOLUX BBS" or "tetrabenzotetraazaporphine". Likewise, in a preferred and non-limiting embodiment,the completed ink composition will contain about 1-200 ppm or about0.0001-0.02% by weight of the invisible dye material (e.g. theuncomplexed invisible metal phthalocyanine far red/infrared fluorophorewith particular reference to chloroaluminum [III] phthalocyaninetetrasulfonic acid and salts thereof). This range is applicable if asingle far red/infrared fluorophore is employed or if multiple farred/infrared fluorophores are used in combination (with the foregoingrange involving the total amount of combined far red/infraredfluorophores in the ink).

The uncompleted invisible metal phthalocyanine far red/infraredfluorophoric dye compositions discussed above (including chloroaluminum[III] phthalocyanine tetrasulfonic acid/salt materials) cannot be seenwith the unaided eye (e.g. are "invisible") as previously noted.However, in accordance with the fluorophoric character thereof, suchmaterials will fluoresce with a high degree of intensity (describedbelow) when illuminated with far red or infrared light (optimally lightwithin a preferred wavelength range of about 650-715 nm). Thisflourescent emission can then be detected and otherwise characterized(observed) using a suitable detection/observation system as outlined inthe next section. Fluorescent emission associated with the foregoinginfrared fluorophores (e.g. the specific and general materials listedherein) will involve the generation of light within the far red orinfrared wavelength regions (optimally within a non-limiting wavelengthrange of about 670-720 nm [particularly when chloroaluminum [III]phthalocyanine tetrasulfonic acid or salts thereof is employed]). Thislight is not visible with the unaided eye and can be analyzed usingsuitable detection devices as indicated below.

The next ingredient of interest to be employed within the inkcomposition associated with this embodiment involves an ink "vehicle"which is primarily used as a carrier medium for the other components inthe completed ink product. In a preferred embodiment, the ink vehiclewill consist of: (1) water; (2) at least one organic ink solventmaterial; or preferably (3) mixtures thereof, with these compositionsbeing present in varied proportions as needed in accordance withpreliminary pilot testing. The term "solvent" as used herein shall bebroadly construed to involve a material that is employed to carry thedesired fluorophore[s] therein in a homogeneous and uniform manner.Likewise, the solvent will also inherently function as a "humectant",namely, a moisture-retaining agent, with the term "solvent" beingconstrued to encompass materials added for solvent purposes, humectantpurposes, or (most often) both purposes. Exemplary and preferred organicink solvents suitable for use in the claimed ink composition include butare not limited to 2-pyrrolidone; ethoxylated glycerol; diethyleneglycol; tetraethylene glycol; 1,5-pentanediol; 1,3-propanediol; N-methylpyrrolidone; 2-propanol; 2-ethyl-2-hydroxymethyl-1,3-propanediol; andmixtures thereof. At this point, it should be emphasized that this inkcomposition and its various alternative embodiments shall not berestricted to any particular ingredients, materials, proportions,amounts, and other parameters unless otherwise stated herein. Anynumerical values presented in this section are provided for examplepurposes only and shall constitute preferred embodiments of theinvention designed to achieve maximum operational efficiency. All of theabove materials may be used in various combinations as determined bypreliminary pilot studies on the ink formulations of concern.

The ink composition associated with this embodiment will basicallyconsist of the selected fluorophore[s] as discussed above and the inkvehicle. Thus, after addition of the fluorophore[s] in the preferredquantity values listed herein (e.g. about 0.0001-0.02% by weight orother values as determined by preliminary testing), the amount of inkvehicle to be combined therewith will be the quantity needed to reach100% by weight. In other words, the ink vehicle will constitute thebalance of the ink composition under consideration above and beyond theamount of fluorophores in use. However, in a preferred embodiment, it ispreferred that the ink composition associated with this embodimentemploy an ink vehicle which is at least about 30% by weight water. Anexemplary ink vehicle will contain about 60-87% by weight water andabout 10-37% by weight of one or more organic solvents, with the balanceinvolving at least one of the other materials listed below.

Next, the ink composition may include a number of optional ingredientsas part of the total ink "vehicle" in varying amounts. For example, anoptional biocide may be added to prevent any microbial growth in thefinal ink product. Exemplary biocides suitable for this purpose includeproprietary products sold under the trademarks PROXEL GXL by ImperialChemical Industries of Manchester, England; UCARCID 250 by Union Carbideof Danbury, Conn. (USA); and NUOSEPT 95 by Huls America, Inc. ofPiscataway, N.J. (USA). If a biocide is used, the final ink compositionwill optimally contain about 0.05-0.5% by weight biocide, with about0.30% by weight being preferred.

Also employed as part of the ink "vehicle" are one or more optionalsurfactant materials which are designed to provide beneficial surfacetension and homogeneity characteristics in the completed inkformulation. While the present invention shall not be restricted to anyparticular surfactant compositions in general (most of which areproprietary in nature), exemplary and preferred materials suitable forthis purpose include but are not limited to a surfactant product soldunder the name "Tergitol 15-S-5" by the Union Carbide Co. of Danbury,Conn. (USA) which basically involves a C₁₁₋₁₅ pareth--5 material havingthe following formula: R₁ R₂ CH(C₂ H₄ O)₅ OH [wherein R₁ and R₂ =C₁₁₋₁₅] and a composition sold under the name "Crodafos N-3 Acid" by Croda,Inc. of Parsippany, N.J. (USA) which comprises a material consisting ofoleth-3 phosphate or polyoxyethylene(3) oleyl ether phosphate (acid).Other surfactant materials suitable for use in the ink composition ofinterest include a compound sold under the name "Mackam OCT-50" byMcIntyre Chemical, Inc. of University Park, Ill. (USA) which basicallyconsists of an octyl betaine composition (e.g. C₈ H₁₇ N⊕CH₃ CH₃ CH₂COO⊖) and a commercial surfactant sold by the Dow Chemical Company ofMidland, Mich. (USA) under the name "Dowfax 8390" which comprises adiphenyl sulfonate (sodium n-hexadecyl diphenyloxide disulfonate)compound. Again, any of these compounds alone or in combination will besuitable for use in the ink composition of interest. While the inksdescribed herein shall not be restricted to any particular quantityvalues in connection with the optional surfactant materials (which mayagain be determined in accordance with routine preliminary pilottesting), it is preferred that the final ink composition contain about0.1-3% by weight total (combined) surfactant therein (if used).

The resulting ink product may be then be used directly in the printingmethods of the present invention as discussed further below. Uponcompletion, the ink composition in this embodiment will typically havean average viscosity of about 1.0-5.0 centipoise, with a surface tensionof about 30-45 dynes/cm although these values are subject to variationin accordance with the specific materials that are selected to producethe final ink formulation. As previously noted, the claimed inkcompositions shall not be restricted to any particular materials ormaterial quantities unless otherwise specified herein. However, thefollowing Example involves a representative and preferred formulationdesigned to provide excellent results and unexpectedly superior imagegeneration using thermal inkjet printing technology:

    ______________________________________                                        Example 1                                                                                              Amount                                               Ingredient               (% by weight)                                        ______________________________________                                        Invisible dye composition (Uncomplexed                                                                 0.007%                                               aluminum phthalocyanine far red/infrared                                                               (e.g. 70 ppm)                                        fluorophore, namely, chloroaluminum [III]                                     phthalocyanine tetrasulfonic acid as                                          illustrated above)                                                            1,5-pentanediol (solvent)                                                                                 8%                                                2-ethyl-2-hydroxymethyl-1,3-propanediol                                                                 7.5%                                                (solvent)                                                                     1,3-propanediol (solvent)                                                                               7.5%                                                Surfactant No. 1 ("TERGITOL 15-S-5" as                                                                  1.5%                                                previously defined)                                                           Surfactant No. 2 ("Dowfax 8390" as previously                                                           1.5%                                                defined)                                                                      Water                    Balance                                                                         100%                                               ______________________________________                                    

Again, this Example is provided for illustration purposes only andrepresents a non-limiting preferred embodiment. In particular, the useof chloroaluminum [III] phthalocyanine tetrasulfonic acid/salts enablesunexpectedly superior results to be achieved and was selected as anoptimum material over a number of other compositions for this reason.Specifically, this dye has excellent water solubility characteristics,high fluorescence, and good lightfastness. Additionally, it avoids thegeneration of weak or misdirected ink droplets in thermal inkjet systemsand likewise avoids the formation of undesired residues therein whichcan lead to cloggage and related problems. This formulation and theother ink compositions described herein are invisible to the unaided eye(e.g. colorless) until they are illuminated with light at an appropriatewavelength as reviewed in the next section.

2. Ink Composition No. 2

This ink composition also comprises a number of ingredients which againcooperate to produce the beneficial results discussed herein. The inkformulation in this embodiment involves all of the ingredients,proportions, physical characteristics, and other parameters mentionedabove in connection with Ink Composition No. 1 unless otherwise noted.In this regard, the foregoing discussion concerning Ink Composition No.1 shall be incorporated by reference in this description of InkComposition No. 2. Nonetheless, in order to provide a full, complete,and enabling disclosure of Ink Composition No. 2 and its functionalattributes, a detailed overview of this material will now be presented.It is important to note that the key distinguishing characteristic ofInk Composition No. 2 over Ink Composition No. 1 involves the use of aplurality of invisible dye materials in Ink Composition No. 2.

The main functional ingredients in Ink Composition No. 2 are multipleinvisible dyes, namely, dyes which cannot be seen by the unaided eyeunder "normal" or "white" light as discussed above, but nonethelessfluoresce when illuminated with light of a specific wavelength. InkComposition No. 2 specifically includes a first invisible dyecomposition comprising an uncomplexed invisible metal phthalocyanine farred/infrared fluorophore as previously defined. Again, "complexed" dyecompositions can present reliability and image-quality problems insystems which, for example, employ thermal inkjet technology on ahigh-speed/high resolution basis (e.g. at least about 600 dpi["dots-per-inch"] at a frequency of about 12-16 kHz or more). The use ofan invisible metal (e.g. aluminum) phthalocyanine fluorophoric dyeproduct that is uncomplexed and employed in a "free" state represents anovel advance in the art of invisible ink imaging, especially inconnection with thermal inkjet technology.

While the present invention in its broadest sense shall not berestricted to any specific uncomplexed invisible metal phthalocyaninefar red/infrared fluorophores, it has been discovered that unexpectedlysuperior results (in terms of image quality, waterfastness,lightfastness, reliability, fluorescence intensity, and the like) areagain achieved in this embodiment through the use of a specialwater-soluble uncomplexed invisible metal (e.g. aluminum) phthalocyaninefar red/infrared fluorophore. This particular material shall bedesignated herein as "chloroaluminum (III) phthalocyanine tetrasulfonicacid" (and salts thereof) which (in the acid form) involves the samestructural formula listed above in connection with Ink Composition No.1.

From a nomenclature standpoint, the foregoing composition again consistsof C₃₂ H₁₆ AlClN₈ O₁₂ S₄, with the following "long-hand" name beingapplicable:chloro[29H,31H-phthalocyanine-2,9,17,24-tetrasulphonato(6-)-N29,N30,N32]-aluminate(4-).In accordance with this formula, four (--SO₃ H) groups are provided. Toform salts of this compound, the hydrogen ions in one or more of the(--SO₃ H) groups (e.g. 1-4 of the groups) may be replaced with apositive counterion preferably selected from the group consisting oflithium (Li⁺) sodium (Na⁺), potassium (K⁺) rubidium (Rb⁺), calcium(Ca⁺²), magnesium (Mg⁺²), aluminum (Al⁺³), ammonium (NH₄ ⁺), andwater-soluble ammonium compounds such as the methyl, ethyl, and ethoxyderivatives thereof. All of the selected counterions may be the samewhen more than one of the (--SO₃ H) groups is involved or mixtures ofdifferent counterions can be employed. A representative and non-limitingexample of a salt of the above-listed composition (namely, sodiumchloroaluminum [III] phthalocyanine tetrasulfonate) was illustratedabove in connection with Ink Composition No. 1 and is equally applicableto Ink Composition No. 2.

Again, many different salts are possible (along with varying"saturation" levels associated with the [SO₃ ⁻¹ ] groups as shownabove). Chloroaluminum (III) phthalocyanine tetrasulfonic acid and saltsthereof are commercially available from the Ciba-Geigy Corp. ofGreensboro, N.C. (USA)/Basel Switzerland under the name "TINOLUX BBS" or"tetrabenzo tetraazaporphine" as previously noted. Likewise, in apreferred and non-limiting embodiment, the completed Ink Composition No.2 will contain about 1-200 ppm or about 0.0001-0.02% by weight of thefirst invisible dye material (e.g. the uncomplexed invisible metal [e.g.aluminum] phthalocyanine far red/infrared fluorophore[s] with particularreference to chloroaluminum [III] phthalocyanine tetrasulfonic acid andsalts thereof). This range is applicable if a single far red/infraredfluorophore is employed or if multiple far red/infrared fluorophores areused in combination (with the foregoing range involving the total amountof combined far red/infrared fluorophores in the ink).

The uncomplexed invisible metal phthalocyanine far red/infraredfluorophoric dye compositions discussed above (including chloroaluminum[III] phthalocyanine tetrasulfonic acid/salt materials) cannot be seenwith the unaided eye (e.g. are "invisible") as previously noted.However, in accordance with the fluorophoric character thereof, suchmaterials will fluoresce with a high degree of intensity (describedbelow) when illuminated with far red or infrared light (optimally lightwithin a preferred wavelength range of about 650-715 nm whichencompasses the far red and infrared wavelengths of primary interest).This flourescent emission can then be detected and otherwisecharacterized (observed) using a suitable detection/observation systemas outlined in the next section. Fluorescent emission associated withthe foregoing far red/infrared fluorophores (e.g. the specific andgeneral materials listed herein) will involve the generation of lightwithin the far red or infrared wavelength regions (optimally within anon-limiting exemplary wavelength range of about 670-720 nm). Thisspecific range is particularly applicable when chloroaluminum [III]phthalocyanine tetrasulfonic acid or salts thereof is employed. Theresulting flourescent light is not visible with the unaided eye and canbe analyzed using suitable detection devices as indicated below.

Ink Composition No. 2 also contains a second invisible dye in order toform a unique FR/IR/UV combination product. The second invisible dyecomprises at least one invisible ultraviolet fluorophore. This materialinvolves a composition which is invisible to the unaided eye as definedabove, but will fluoresce when illuminated with ultraviolet light.Ultraviolet light traditionally involves a wavelength range of about250-400 nm, with the light to be applied in this embodiment optimallybeing within a non-limiting wavelength range of about 250-380 nm. Theresulting flourescent light generated by the ultraviolet fluorophore inthe present embodiment will have an optimal, non-limiting wavelengthrange of about 400-650 nm which is visible to the unaided eye. Manydifferent materials may be employed as the invisible ultravioletfluorophore without limitation provided that they retain the functionalcharacteristics listed above. However, exemplary and preferredultraviolet fluorophores suitable for this purpose include but are notlimited to ultraviolet absorbing materials in the following classes:stilbenes, pyrazolines, coumarins, carbostyrils, pyrenes, and mixturesthereof. Representative materials in each of these classes are asfollows: (1) stilbenes:4,4'-bis(triazin-2-ylamino)stilbene-2,2'-disulfonic acid;benzenesulfonicacid-2,2'-(1,2-ethenediyl)bis[5-[4-bis(2-hydroxyethyl)amino]-6-[(4-sulfophenyl)amino]-1,3,5-triazin-2yl]amino-tetrasodiumsalt; and4,4'-bis[4-diisopropanolamino-6-(p-sulfoanilino)-s-triazin-2-yl-amine]stilbene-sodiumdisulfonate; (2) pyrazolines: 1,2-diphenyl-2-pyrazoline; (3) coumarins:7-diethylamino-4-methylcoumarin; 7-hydroxy-4-methylcoumarin; and3-(2-benzimidazolyl)-7-(diethylamino)coumarin; (4) carbostyrils:2-hydroxyquinoline; and (5) pyrenes: N-(1-pyrenebutanoyl)cysteic acid.Also of interest as an ultraviolet fluorophore isdibenzothiophene-5,5-dioxide, as well as C.I. (Color Index) FluorescentBrightener 28; C.I. Fluorescent Brightener 220; and C.I. FluorescentBrightener 264, with some or all of these C.I. compositions beingcomparable or structurally equivalent to the specific materials listedabove. The foregoing ultraviolet fluorophores and others arecommercially available from numerous sources including but not limitedto the Aldrich Chemical Co. of Milwaukee, Wis. (USA); Bayer Corporationof Pittsburgh, Pa. (USA) under the names "BLANKOPHORE" or "PHORWHITE";Ciba-Geigy Corporation of Greensboro, N.C. (USA)/Basil, Switzerland;Molecular Probes of Eugene, Oreg. (USA); Sandoz Chemicals of Charlotte,N.C. (USA) under the name "LEUKOPHOR"; and Sigma Co. of St. Louis, Mo.(USA). The list provided above shall be considered representative onlyand non-limiting. As previously mentioned, the selected ultravioletfluorophore is invisible when viewed by the unaided eye in "normal" or"white" light, but will fluoresce when illuminated with ultravioletlight (ideally within the foregoing wavelength range.) Likewise, thefluorescent emission provided by the ultraviolet fluorophore[s] in apreferred embodiment (including the ultraviolet fluorophores recitedabove) will be readily viewable with the unaided eye. As noted above,they will optimally emit light within a non-limiting wavelength range ofabout 400-650 nm which will enable them to be viewed without the needfor special monitoring and detecting equipment.

The use of a "dual" fluorophoric invisible ink composition whichcontains both ultraviolet and far red/infrared fluorophores incombination offers many benefits. For example, it provides amulti-functional invisible imaging system with varying security levels.The primary attributes of such a system involve (1) substantiallyincreased flexibility in enabling the use of either a far red/infraredor ultraviolet reader unit with a single ink composition; and (2) anenhanced level of security by requiring readability in two differentwavelength regions. Accordingly, Ink Composition No. 2 represents aconsiderable advance in the art of invisible ink technology, withparticular reference to the "dual" fluorophoric nature of the ink.

While Ink Composition No. 2 shall not be restricted to any particularnumerical parameters in connection with the ingredients used therein,the completed ink formulation will preferably contain about 500-50000ppm or about 0.05-5% by weight of the second invisible dye material(ultraviolet fluorophore). This range is applicable if a singleultraviolet fluorophore is employed or if multiple ultravioletfluorophores are used in combination (with the foregoing range involvingthe total amount of combined ultraviolet fluorophores in the ink).

The remaining ingredients employed in Ink Composition No. 2 aresubstantially the same as those listed above in connection with InkComposition No. 1. However, to again facilitate a full and completedisclosure of the claimed technology, a detailed summary of theseingredients will now be provided. The next ingredient of interest to beemployed within Ink Composition No. 2 involves an ink "vehicle" which isprimarily used as a carrier medium for the other components in thecompleted ink product. In a preferred embodiment, the ink vehicle willcontain (1) water; (2) at least one organic ink solvent material; orpreferably (3) mixtures thereof, with these compositions being presentin varied proportions as needed in accordance with preliminary pilottesting. The term "solvent" as used herein shall be construed to broadlyinvolve a material that is employed to carry the desired fluorophore[s]therein in a homogeneous and uniform manner. Likewise, the solvent willalso inherently function as a "humectant", namely, a moisture-retainingagent, with the term "solvent" being construed to encompass materialsadded for solvent purposes, humectant purposes, or most often both ofthese purposes. Exemplary and preferred organic ink solvents suitablefor use in Ink Composition No. 2 include but are not limited to2-pyrrolidone; ethoxylated glycerol; diethylene glycol; tetraethyleneglycol; 1,5-pentanediol; 1,3-propanediol; N-methyl pyrrolidone;2-propanol; 2-ethyl-2-hydroxymethyl-1,3-propanediol; and mixturesthereof. At this point, it should be emphasized that this inkcomposition and its various alternative embodiments shall not berestricted to any particular ingredients, materials, proportions,amounts, and other parameters unless otherwise stated herein. Anynumerical values presented in this section are provided for examplepurposes only and shall constitute preferred embodiments of theinvention designed to achieve maximum operational efficiency. All of theabove materials may be used in various combinations as determined bypreliminary pilot studies on the ink formulations of concern. The inkcomposition associated with this embodiment will basically consist ofthe selected fluorophore[s] as discussed above and the ink vehicle.Thus, after addition of both fluorophore[s] in the preferred quantityvalues listed herein (e.g. about 0.0001-0.02% by weight for the farred/infrared fluorophore[s], about 0.05-5% by weight for the ultravioletfluorophore[s], or other values as determined by preliminary testing),the amount of ink vehicle to be combined therewith will be the quantityneeded to reach 100% by weight. In other words, the ink vehicle willconstitute the balance of Ink Composition No. 2 above and beyond theamount of fluorophores in use. However, in a preferred embodiment, it ispreferred that the ink composition associated with this embodimentemploy an ink vehicle which is at least about 30% by weight water. Anexemplary ink vehicle will contain about 60-87% by weight water andabout 10-37% by weight of one or more organic solvents, with the balanceinvolving at least one of the other materials listed below.

Ink Composition No. 2 may also include a number of optional ingredientsas part of the total ink "vehicle" in varying amounts. For example, anoptional biocide may be added to prevent any microbial growth in thefinal ink product. Exemplary biocides suitable for this purpose againinclude proprietary products sold under the trademarks PROXEL GXL byImperial Chemical Industries of Manchester, England; UCARCID 250 byUnion Carbide of Danbury, Conn. (USA); and NUOSEPT 95 by Huls America,Inc. of Piscataway, N.J. (USA). If a biocide is used, the final inkcomposition will optimally contain about 0.05-0.5% by weight biocide,with about 0.30% by weight being preferred.

Also employed as part of the ink "vehicle" are one or more optionalsurfactant materials which are designed to provide beneficial surfacetension and homogeneity characteristics in the completed ink product.While the present invention shall not be restricted to any particularsurfactants in general (most of which are proprietary in nature),exemplary and preferred materials suitable for this purpose include butare not limited to a surfactant product sold under the name "Tergitol15-S-5" by the Union Carbide Co. of Danbury, Conn. (USA) which basicallyinvolves a C₁₁₋₁₅ pareth--5 material having the following formula: R₁ R₂CH(C₂ H₄ O)₅ OH [wherein R₁ and R₂ =C₁₁₋₁₅ ] and a composition soldunder the name "Crodafos N-3 Acid" by Croda, Inc. of Parsippany, N.J.(USA) which comprises a material consisting of oleth-3 phosphate orpolyoxyethylene(3) oleyl ether phosphate (acid). Other surfactantmaterials suitable for use in Ink Composition No. 2 include a productsold under the name "Mackam OCT-50" by McIntyre Chemical, Inc. ofUniversity Park, Ill. (USA) which basically consists of an octyl betainecomposition (e.g. C₈ H₁₇ N⊕CH₃ CH₃ CH₂ COO⊖) and a commercial surfactantsold by the Dow Chemical Company of Midland, Mich. (USA) under the name"Dowfax 8390" which comprises a diphenyl sulfonate (sodium n-hexadecyldiphenyloxide disulfonate) compound. Again, any of these compounds aloneor in combination will be suitable for use in Ink Composition No. 2.While the inks described herein shall not be restricted to anyparticular quantity values in connection with the optional surfactants(which may again be determined in accordance with routine preliminarytesting), Ink Composition No. 2 will preferably contain about 0.1-3% byweight total (combined) surfactant therein.

The resulting ink product may be then be used directly in the methods ofthe present invention as discussed further below. Upon completion, theink composition in this embodiment will typically have an averageviscosity of about 1.0-5.0 centipoise, with a surface tension of about30-45 dynes/cm although these values are subject to variation inaccordance with the specific materials that are selected to produce thefinal ink formulation. As previously noted, the ink compositions ofinterest as described herein shall not be restricted to any particularmaterials or material quantities unless otherwise specified. However,the following Example involves a representative and preferred InkComposition No. 2 which is designed to provide excellent results andunexpectedly superior image generation using thermal inkjet printingtechnology:

    ______________________________________                                        Example 2                                                                                               Amount                                              Ingredient                (% by weight)                                       ______________________________________                                        Invisible dye composition (Uncomplexed                                                                  0.007%                                              aluminum phthalocyanine far red/infrared                                                                (e.g. 70 ppm)                                       fluorophore, namely, chloroaluminum [III]                                     phthalocyanine tetrasulfonic acid as                                          illustrated above)                                                            Second invisible dye composition (an ultraviolet                                                           3%                                               fluorophore, namely, benzenesulfonic acid-2,2'-                               amino]-6-[(4-sulfophenyl)amino]-1,3,5-triazin-                                2yl]amino-tetrasodium salt sold under the name                                "BLANKOPHOR P" by Bayer Corp. of Pittsburg, PA                                [USA]).                                                                       1,5-pentanediol (solvent)    8%                                               2-ethyl-2-hydroxymethyl-1,3-propanediol                                                                  7.5%                                               (solvent)                                                                     1,3-propanediol (solvent)  7.5%                                               Surfactant No. 1 ("TERGITOL 15-S-5" as                                                                   1.5%                                               previously defined)                                                           Surfactant No. 2 ("Dowfax 8390" as previously                                                            1.5%                                               defined)                                                                      Water                     Balance                                                                         100%                                              ______________________________________                                    

Again, this Example is provided for illustration purposes only andrepresents a non-limiting preferred embodiment. In particular, the useof chloroaluminum [III] phthalocyanine tetrasulfonic acid/salts aspreviously discussed enables unexpectedly superior results to beachieved and was selected as an optimum material over a number of othercompositions for this reason. A specific discussion of the benefitsprovided by the use of this product is presented above in connectionwith Ink Composition No. 1 and is incorporated herein by reference.

Having set forth the preferred and novel ink formulations of interest,the unique imaging methods associated therewith will now be discussed.Again, the claimed methods and materials have widespread applicabilityto many different printing technologies although, as previously stated,they are particularly useful in systems which employ thermal inkjettechnology. Thus, the following section shall emphasize the use ofthermal inkjet printing techniques with the understanding that theinventive concepts presented herein shall not be restricted to thisparticular method.

C. Printing and Detecting Methods

1. The Generation of Invisible Printed Images Using Ink Composition No.1

With reference to FIG. 2, a representative method is schematicallyillustrated for generating an invisible printed image on a substratewhich can thereafter be detected on-demand. While the followingdiscussion shall again involve an inkjet printing system (which employsthermal inkjet technology), many other printing methods may be used todeliver the claimed ink compositions including those ranging fromconventional silk screening operations to standard offset printingtechniques.

As schematically shown in FIG. 2, an inkjet printing unit 200 isprovided which is employed as the printing apparatus in this embodiment.Many different systems may be selected for use as the printing unit 200,including printers manufactured and sold by the Hewlett-Packard Companyof Palo Alto, Calif. (USA) under the following product designations:DESKJET 400C, 500C, 540C, 660C, 693C, 820C, 850C, 870C, 1200C, and1600C. In addition, the inkjet printing unit 200 may consist of aspecialized system described in co-owned and co-pending U.S. patentapplication Ser. No. 09/181,589 filed concurrently herewith on Oct. 28,1998 (HP Case No. 10981510-1). A thermal inkjet cartridge unit (e.g.cartridge 10 illustrated in FIG. 1 and discussed above) is providedwithin the printing unit 200 which is supplied with a selected inkcomposition 32. The thermal inkjet cartridge 10 employed in thisembodiment shall be characterized herein as an "ink delivery system" inview of its ability to selectively deliver the ink composition 32 to achosen substrate on-demand. The ink composition 32 shown in FIG. 2consists of invisible Ink Composition No. 1 as previously describedwhich contains at least one uncomplexed invisible metal phthalocyaninefar red/infrared fluorophore (particularly chloroaluminum [III]phthalocyanine tetrasulfonic acid and salts thereof). Again, manydifferent inkjet and non-inkjet cartridge types (e.g. "ink deliverysystems") may be employed in the processes described herein, with thesesystems including a housing, a printhead in fluid communication with thehousing (which contains an ink storage/retaining chamber therein), andat least one ink ejector in the printhead. Regarding specific thermalinkjet cartridges which can be used as the cartridge unit 10, theinvention shall not be limited in this regard. For example, numerouscommercially-available cartridge units which are suitable for thispurpose include those produced by the Hewlett-Packard Company of PaloAlto, Calif. (USA) under the following product designations: 51641A,51645A, 51640C, 51640A, 51629A, and 51649A. All of the methods discussedherein shall likewise be equally applicable to ink delivery systemswhich employ (1) a printhead directly attached to the ink-containinghousing; and (2) a printhead connected via one or more tubular conduitsto a remotely-positioned housing having an ink supply therein aspreviously noted. The latter system is known as an "off-axis" unit asdiscussed in, for example, co-owned Pending U.S. patent application Ser.No. 08/869,446 filed on Jun. 5, 1997 which is incorporated herein byreference. The invention shall therefore not be restricted to anyparticular ink delivery systems, with many different variants beingapplicable.

With continued reference to FIG. 2, a substrate 202 is provided andinserted (e.g. placed) into the printing unit 200. The top surface 204of the substrate 202 faces upwardly toward the ink cartridge 10. Manydifferent materials may be used to produce the substrate 202 including,without limitation, paper (coated or uncoated), metal, plastic filmmaterials (made of, for example, polyester resins, polycarbonateproducts, polyethylene compounds, and others), glass, and the like. Inthis regard, the claimed ink compositions and printing methods shall benot restricted to any given substrate materials which may be employed insheet, strip, roll, or other forms without limitation. However, theprocesses shown in FIGS. 2-3 are particularly useful in connection withpaper-based financial documents including but not limited to checks,statements, insurance papers, routing materials, and the like.

As schematically illustrated in the embodiment of FIG. 2, the printingunit 200 is electrically connected to an image generating apparatus 208which may involve many different systems. Representative systems areselected from the group consisting of a personal computer (e.g. of thetype manufactured by the Hewlett-Packard Company of Palo Alto, Calif.[USA] under the trademark "PAVILION®"), a scanner unit (e.g. of thevariety sold by the Hewlett-Packard Company of Palo Alto, Calif. [USA]under the trademark "SCANJET®"), both of these devices, or any othersuitable image producing system (including a bar-code generator) whichwill vary depending on the intended use of the ink compositions. Theclaimed processes shall likewise not be restricted to any particularimage generation device, protocol, or format.

Next, the image generating apparatus 208 and the printing unit 200 arecooperatively activated in order to deliver the desired invisibleprinted image 210 (shown in phantom lines in FIG. 2) onto the topsurface 204 of the substrate 202. The printed image 210 can involve abar-code as illustrated in FIG. 2 or any other desired indicia. Both theimage generating apparatus 208 and the printing unit 200 are used toselectively control the ink cartridge 10. The printing process isinitiated by activation of the ink ejectors (e.g. thin-film resistors86) in the printhead 80 of the ink cartridge 10 (See FIG. 1). The term"activation" in the system of FIG. 2 shall again involve a procedure inwhich the thin-film resistors 86 of the ink cartridge 10 are directed bythe printing unit 200 to deliver the ink composition 32 from thecompartment/chamber 30 of the housing 12 onto the top surface 204 of thesubstrate 202. This is accomplished by energizing the thin filmresistors 86 in the printhead 80 of the cartridge 10. As a result, inkresiding beneath the orifice plate 104 is thermally excited and expelledoutwardly through the ink ejection orifices 108 in the plate 104 andonto the image-receiving substrate 202. In this manner, the cartridge 10may be used to deliver the invisible printed image 210 to the substrate202 using ink composition 32. The invisible printed image 210 ischaracterized as being "invisible" because it cannot be viewed by theunaided eye in "normal", "ambient", or "white" light as discussed above.Likewise, in accordance with the unique and specialized chemicalcomponents within the ink composition 32 (namely, Ink Composition No.1), the invisible printed image 210 is stable (e.g. waterfast/lightfast)with a high resolution capacity. Again, the materials and processesdiscussed herein are appropriate for at least 600 dpi printing at afrequency of about 12-16 kHz or more.

At this point, the substrate 202 containing the invisible printed image210 is preferably removed from the printing unit 200. When desired, theprinted image 210 can be detected and otherwise characterized. Toachieve this goal, the invisible printed image 210 produced from the inkcomposition 32 is illuminated by applying light at a wavelengthsufficient to cause the printed image 210 to generate fluorescent light.This is accomplished in the present embodiment by delivering far red orinfrared light onto the image 210. Far red light normally involves awavelength range of about 650-700 nm, with infrared light involving arange which exceeds about 700 nm up to about 1000 nm. However, in apreferred and non-limiting embodiment designed to produce optimumresults, the applied light is preferably within a wavelength range ofabout 650-715 nm (with a range of about 660-690 nm being best). Althoughthe invisible printed image 210 has some absorption in the far redportion of the visible spectrum as noted above, the image 210 is againnot detectible by the unaided eye because of the low dye concentrationsbeing used and the low sensitivity of the human eye to wavelength valuesin this region.

As a result of this step, the ink composition 32 and invisible printedimage 210 produced therefrom will emit far red or infrared fluorescentlight. This light will optimally be within a non-limiting wavelengthrange of about 670-720 (preferred=about 670-710 nm) when the chemicalmaterials discussed herein are used (including but not limited tochloroaluminum [III] phthalocyanine tetrasulfonic acid or saltsthereof.) A far red/infrared illumination system 212 is schematicallyillustrated in FIG. 2 which may be used to deliver far red or infraredlight 214 within the foregoing wavelength parameters to the substrate202 and invisible printed image 210. Many different light sources may beemployed in connection with the illumination system 212 (includingstandard red LED ["light-emitting diode"] light delivery systems,halogen bulb illuminators, metal halide bulb units, and other comparablesystems known in the art for infrared imaging). While this embodimentshall not be restricted to any particular systems for this purpose,exemplary commercial illuminators which may be employed as theillumination system 212 include products sold by Micro Laser Systems,Inc. of Garden Grove, Calif. (USA)--model L4 780s-24; IlluminationTechnologies, Inc. of Syracuse, N.Y. (USA)--model 3900; and Nikon ofJapan under the designation "Metal Halide Fiber Optic Illuminator".

When the illumination system 212 is used to deliver light 214 to theinvisible printed image 210, it will fluoresce to produce a fluorescentprinted image designated at reference number 218 in FIG. 2. However, thefluorescent printed image 218 will not fluoresce in a manner which isvisible to the unaided eye. Instead, it will fluoresce by generating farred or infrared light within an optimal, non-limiting wavelength rangeof about 670-720 nm (best=about 670-710 nm) in accordance with thespecific materials used to produce the ink composition 32 including themetal phthalocyanine far red/infrared fluorophore, with particularreference to chloroaluminum (III) phthalocyanine tetrasulfonic acid andsalts thereof. To detect or otherwise characterize the fluorescentprinted image 218 on the top surface 204 of the substrate 202, anappropriate detecting system 220 is provided. The detecting system 220may involve many different devices and components without limitation.For example, the system 220 schematically shown in FIG. 2 consists of astandard CCD ("charge coupled device") camera 222 which is fitted withan appropriate infrared filter 224 of known construction (e.g. aconventional 700 nm long pass filter in a representative, non-limitingexample) Other camera systems are also suitable for use herein, with theforegoing arrangement of components being provided for example purposesonly. With continued reference to FIG. 2, representativecommercially-available detection devices which may be used in connectionwith the camera 222 include but are not limited to camera systems soldby the Pulnix Co. of Sunnyvale, Calif. (USA)--model 440 and GlobalSupply Co. of Toronto, Canada--model UNI-IR5.

The specific embodiment of FIG. 2 is particularly designed to read andotherwise detect bar-code images. Once the camera 222 is activated, theresulting electronic images are directed into an image processor 230schematically illustrated in FIG. 2. The image processor 230 can involvemany different systems without limitation including those which aredesigned to detect, interpret, and characterize bar code information. Anexemplary commercial apparatus which is suitable for this purposeincludes a system produced by Accusort, Inc. of Telford, Pa.(USA)--model 24. Likewise, many different devices may be used inconnection with the image processor 230 ranging from bar code readers aspreviously discussed to video monitors which are designed to producevisually-observable images from signals generated by the camera 222. Thepresent invention shall therefore not be restricted to any particularsystems, sub-systems, or components associated with the image processor230 which will vary depending on the intended use of the inkformulations. Regardless of which components are employed within thedetecting system 220, it will enable the fluorescent printed image 218to be detected on-demand in a highly effective manner, thus completingthe image generating and monitoring process.

The production of invisible printed images in accordance with thisembodiment (which employs Ink Composition No. 1) enables clear andstable printed images to be generated. Of particular value is theability to accomplish these goals using many different printing systemsincluding high-speed thermal inkjet units. The resulting images areagain characterized by high resolution and throughput, excellentstability (including waterfastness and lightfastness), and the abilityto fluoresce with a considerable degree of intensity. As a final noteregarding the system of FIG. 2, the configuration of components showntherein is schematic only and may be varied as needed. A specificexample of an integrated system which may be used in connection with theembodiment of FIG. 2 is again disclosed in co-owned, co-pending U.S.patent application Ser. No. 09/181,589 (filed concurrently on Oct. 28,1998)--(HP Case No. 10981510-1) which is incorporated herein byreference.

2. The Generation of Invisible Printed Images Using Ink Composition No.2

With reference to FIG. 3, another method is schematically illustratedfor generating an invisible printed image on a substrate which canthereafter be detected, viewed, or otherwise characterized on-demand.This method uses Ink Composition No. 2 discussed above which employs adual fluorophore system, namely, (1) an ultraviolet fluorophore; and (2)a far red/infrared fluorophore in combination. Again, while thefollowing discussion shall primarily involve an inkjet printing system(which employs thermal inkjet technology), many other printingtechniques can be used in connection with the ink compositions ofinterest including the alternatives mentioned above.

It shall be understood that the method associated with this embodimentwill employ the same steps listed above in connection with the priorembodiment except for one item, namely, the manner in which theinvisible printed image is detected. Regarding the preliminary stages ofthe claimed process which occur prior to viewing or otherwise detectingthe invisible printed image, the information presented above inconnection with the first embodiment shall be incorporated by referencein this section. The use of common reference numbers in FIGS. 2-3 willsignify process steps and materials that are equally applicable to bothembodiments. However, to provide a full and enabling disclosure, a briefoverview of the claimed procedure from start to finish will now beprovided.

With reference to FIG. 3, a thermal inkjet printing unit 200 is againillustrated which is used as the printing apparatus in this embodiment.Exemplary commercial systems associated with the printing unit 200 arediscussed above. A thermal inkjet cartridge unit (e.g. cartridge 10shown in FIG. 1) is likewise positioned within the printing unit 200which is supplied with an ink composition 32. In this embodiment, theink composition 32 will involve Ink Composition No. 2 which includes asactive ingredients (1) at least one uncomplexed invisible metalphthalocyanine far red/infrared fluorophore (particularly chloroaluminum[III] phthalocyanine tetrasulfonic acid or salts thereof which provideunexpectedly superior results); and (2) at least one invisibleultraviolet fluorophore (with examples thereof being presented above inSection "B"). Again, many different inkjet and non-inkjet cartridgetypes (e.g. "ink delivery systems") may be employed in the processesdescribed herein, with these systems including a housing, a printhead influid communication with the housing (which contains an inkstorage/retaining chamber therein), and at least one ink ejector in theprinthead. Regarding specific thermal inkjet cartridges which can beused as the ink cartridge unit 10 in this embodiment, the inventionshall not be limited in this respect. Representativecommercially-available cartridge units suitable for this purpose are thesame as those listed above in connection with the first embodiment. Allof the methods discussed herein shall likewise be equally applicable toink delivery systems which employ (1) a printhead directly attached tothe ink-containing housing; and (2) a printhead operatively connectedvia one or more tubular conduits to a remotely-positioned housing havingan ink supply therein as noted above.

Next, a substrate 202 of the type previously discussed is provided andinserted (e.g. placed) into the printing unit 200. The top surface 204of the substrate 202 faces upwardly toward the ink cartridge 10. Manydifferent materials may be used to produce the substrate 202 including,without limitation, paper (coated or uncoated), metal, plastic filmmaterials (made of, for example, polyester resins, polycarbonateproducts, polyethylene compounds, and others), glass, and the like. Inthis regard, the ink compositions and methods associated with thisembodiment shall be not restricted to any given substrate materialswhich may again be employed in sheet, strip, roll, or other formswithout limitation. As schematically illustrated in the embodiment ofFIG. 3, the printing unit 200 is electrically connected to an imagegenerating apparatus 208 which may involve many different systems.Representative systems which can be used as the image generatingapparatus 208 are the same as those listed above in connection with thefirst embodiment including personal computers, scanners, bar codegenerators, and the like.

The image generating apparatus 208 and the printing unit 200 are thencooperatively activated in order to deliver the desired invisibleprinted image 210 (shown in phantom lines in FIG. 3) onto the topsurface 204 of the substrate 202. The printed image 210 can involve abar-code as illustrated in FIG. 3 or any other desired indicia. Both theimage generating apparatus 208 and the printing unit 200 are used toselectively control the ink cartridge 10. The printing process isinitiated by activation of the ink ejectors (e.g. thin-film resistors86) in the printhead 80 of the ink cartridge 10 (See FIG. 1). The term"activation" in the system of FIG. 3 shall again involve a process inwhich the thin-film resistors 86 of the ink cartridge 10 are directed bythe printing unit 200 to deliver the ink composition 32 from thecompartment/chamber 30 of the housing 12 onto the top surface 204 of thesubstrate 202. This is achieved by energizing the thin film resistors 86in the printhead 80 of the cartridge 10. As a result, ink residingbeneath the orifice plate 104 is thermally excited and expelledoutwardly through the ink ejection orifices 108 in the plate 104 andonto the image-receiving substrate 202. In this manner, the cartridge 10may be used to deliver the invisible printed image 210 to the substrate202 using ink composition 32. The invisible printed image 210 ischaracterized as being "invisible" because it again cannot be viewed bythe unaided eye in "normal", "ambient", or "white" light as previouslydefined. Likewise, in accordance with the unique and specializedchemical components within the ink composition 32 (namely, InkComposition No. 2), the invisible printed image 210 is stable (e.g.waterfast/lightfast) with a high resolution level. Again, the materialsand processes discussed in this alternative embodiment are appropriatefor at least 600 dpi printing at a frequency of about 12-16 kHz or more.

At this point, the substrate 202 containing the invisible printed image210 thereon is preferably removed from the printing unit 200 andilluminated with light having a wavelength sufficient to cause the inkcomposition 32 to emit fluorescent light therefrom. The system operatorin the present embodiment now has a choice as to the type and manner ofillumination which will be used to accomplish illumination/detection.The first option involves a procedure which is substantially the same asthe process described above in the first embodiment. Specifically, theinvisible printed image 210 produced from the ink composition 32 isilluminated by applying far red or infrared light onto the printed image210 to generate fluorescent light. Far red light normally involves awavelength range of about 650-700 nm, with infrared light involving arange which exceeds about 700 nm up to about 1000 nm as noted above.However, in a preferred and non-limiting embodiment designed to produceoptimum results, the applied light is preferably within a wavelengthrange of about 650-715 nm (with a range of about 660-690 nm being best).As a result of this step, the ink composition 32 and invisible printedimage 210 produced therefrom will emit far red or infrared fluorescentlight. This light will optimally be within a non-limiting wavelengthrange of about 670-720 (preferred=about 670-710 nm) when the chemicalmaterials discussed herein are used (including but not limited tochloroaluminum [III] phthalocyanine tetrasulfonic acid or saltsthereof.) A far red/infrared illumination system 212 is schematicallyillustrated in FIG. 3 which may be employed to deliver far red/infraredlight 214 within the foregoing wavelength range to the substrate 202 andinvisible printed image 210. Many different light sources may be used inconnection with the illumination system 212 (including standard red LED["light-emitting diode"] light delivery systems, halogen bulbilluminators, metal halide bulb units, and other comparable systemswhich are known in the art for infrared imaging). While this embodimentshall not be restricted to any particular systems for this purpose,exemplary commercial illuminators which can be employed as theillumination system 212 include the specific products mentioned above inconnection with the previous embodiment.

When the illumination system 212 is used to deliver far red/infraredlight 214 to the invisible printed image 210 on the substrate 202, itwill fluoresce to produce a fluorescent printed image designated atreference number 218 in FIG. 3. However, the fluorescent printed image218 will not fluoresce in a manner which is visible to the unaided eye.Instead, it will fluoresce by producing far red or infrared light withinan optimal, non-limiting wavelength range of about 670-720 nm(best=about 670-710 nm) in accordance with the specific materials usedto produce the ink composition 32 including the metal phthalocyanine farred/infrared fluorophore, with particular reference to chloroaluminum(III) phthalocyanine tetrasulfonic acid and salts thereof. To detect orotherwise characterize the fluorescent printed image 218 on the topsurface 204 of the substrate 202, an appropriate detecting system 220 isprovided. The detecting system 220 may involve many different devicesand components without limitation. For example, the system 220schematically shown in FIG. 3 again consists of a standard CCD ("chargecoupled device") camera 222 which is fitted with an appropriate infraredfilter 224 of known construction (e.g. a conventional 700 nm long passfilter in a representative, non-limiting example). Other camera systemsare also suitable for use herein, with the foregoing arrangement ofcomponents being provided for example purposes only. Representativecommercially-available detection devices which may be used in connectionwith the camera 222 include but are not limited to thecommercially-available camera units discussed above in connection withthe first embodiment.

The specific embodiment of FIG. 3 is particularly designed to read andotherwise detect bar-code images. Once the camera 222 is activated, theresulting electronic images are directed into an image processor 230schematically shown in FIG. 3. The image processor 230 can involve manydifferent systems without limitation including those which are designedto detect, interpret, and characterize bar code information. Anexemplary commercial apparatus which is suitable for this purposeincludes the same representative system previously described inconnection with the prior embodiment. Likewise, many different devicesmay be used as the image processor 230 ranging from bar code readers tovideo monitors which are designed to produce visually-observable imagesfrom signals generated by the camera 222. Regardless of which componentsare employed within the detecting system 220, it will enable thefluorescent printed image 218 to be detected on-demand in a highlyeffective manner, thus completing the far red/infrared image generatingand monitoring process.

The production of invisible printed images in accordance with thisembodiment (which employs Ink Composition No. 2) enables clear andstable printed images to be generated. Of particular value is theability to accomplish these goals using many different printing systemsincluding high-speed thermal inkjet units. The resulting images in thisalternative embodiment are again characterized by high resolution andthroughput, excellent stability (including waterfastness andlightfastness), and the ability to fluoresce with a considerable degreeof intensity.

In accordance with the unique dual-fluorophore character of InkComposition No. 2, the invisible printed image 210 can likewise beviewed by ultraviolet illumination (FIG. 3). To accomplish this goal,the invisible printed image 210 produced from the ink composition 32 isilluminated by applying ultraviolet light onto the image 210.Ultraviolet light, as noted above, traditionally involves a wavelengthrange of about 250-400 nm, with the light to be applied in thisembodiment optimally being within a non-limiting wavelength range ofabout 250-380 nm. With continued reference to FIG. 3, an ultravioletillumination system 240 is schematically illustrated which may be usedto deliver ultraviolet light 242 to the substrate 202 and printed image210. Many different devices may be employed in connection with theultraviolet illumination system 212 including but not limited to knownultraviolet illuminators and/or conventional "blacklight" systems.Representative commercially available ultraviolet devices suitable foruse in this embodiment include illuminators sold by the Cole-Parmer Co.of Vernon Hills, Ill. (USA)--model UVGL-58; and Nikon of Japan under thename "Metal Halide Fiber Optic Illuminator".

To view the fluorescent printed image (designated at reference number244 in FIG. 3) on the top face 204 of the substrate 202, no specialequipment is required. This situation exists because the light emittedby the fluorescent printed image 244 will have an optimal, non-limitingwavelength of about 400-650 nm which is, in effect, visible to theunaided eye without special observation equipment. Likewise, inaccordance with the visible nature of the printed image 244, it may alsobe viewed with a conventional video camera system (not shown).

The production of invisible printed images using this particularembodiment (which employs Ink Composition No. 2) provides the samebenefits listed above, along with the generation of clearly definedultraviolet images. Of particular value is the ability to accomplishthese goals using many different printing systems including high-speedthermal inkjet units. The resulting images are again characterized byhigh resolution and throughput, excellent stability (includingwaterfastness and lightfastness), and the ability to fluoresce with aconsiderable degree of intensity.

As described herein, the claimed ink compositions and printing methodsovercome numerous problems associated with prior invisible ink systemsand provide many advantages including but not limited to (1) high printquality levels (particularly when thermal inkjet technology isemployed); (2) superior lightfastness and waterfastness; (3) excellentfluorescence intensity during illumination with an appropriate lightsource; and (4) a high level of reliability when used in connection withinkjet printing systems (particularly those which employ thermal inkjettechnology). Accordingly, the present invention represents an advance ininvisible ink imaging technology which satisfies a long-felt need asnoted herein.

Having set forth preferred embodiments of the invention, it isanticipated that suitable modifications may be made thereto byindividuals skilled in the relevant art which nonetheless remain withinthe scope of the invention. For example, the invention shall not belimited to any particular ink formulations, printing equipment, appliedlight wavelength ranges, emitted light wavelength ranges, and imagingsteps within the general parameters set forth above unless otherwisenoted. In this regard, the invention shall only be construed inaccordance with the following claims:

The invention that is claimed is:
 1. An invisible ink composition comprising:an invisible dye comprised of at least one uncompleted invisible metal phthalocyanine fluorophore which absorbs light within a wavelength range of about 650-715 nm and emits light within a wavelength range of about 670-720 nm, said ink composition comprising about 0.0001-0.02% by weight said phthalocyanine fluorophore; and an ink vehicle comprised of water and at least one organic solvent.
 2. An invisible ink composition comprising:an invisible dye comprised of a phthalocyanine fluorophore selected from the group consisting of chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereof; and an ink vehicle comprised of water and at least one organic solvent.
 3. The invisible ink composition of claim 2, wherein said ink composition comprises about 0.0001-0.02% by weight said phthalocyanine fluorophore.
 4. The invisible ink composition of claim 2 wherein said phthalocyanine fluorophore absorbs light within a wavelength range of about 650-715 nm and emits light within a wavelength range of about 670-720 nm.
 5. An invisible ink composition comprising a phthalocyanine fluorophore selected from the group consisting of chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereof, 1,5-pentanediol, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 1,3-propanediol, at least one surfactant, and water.
 6. An invisible ink composition comprising:a first invisible dye comprised of at least one uncompleted invisible metal phthalocyanine fluorophore which absorbs light within a wavelength range of about 650-715 nm and emits light within a wavelength range of about 670-720 nm; a second invisible dye comprised of at least one invisible ultraviolet fluorophore; and an ink vehicle comprised of water and at least one organic solvent.
 7. The invisible ink composition of claim 6 wherein said ink composition comprises about 0.0001-0.02% by weight said phthalocyanine fluorophore and about 0.05-5% by weight said ultraviolet fluorophore.
 8. The invisible ink composition of claim 6 wherein said ultraviolet fluorophore absorbs light within a wavelength range of about 250-380 nm and emits light within a wavelength range of about 400-650 nm.
 9. An invisible ink composition comprising:a first invisible dye comprised of a phthalocyanine fluorophore selected from the group consisting of chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereof; a second invisible dye comprised of at least one ultraviolet fluorophore; and an ink vehicle comprised of water and at least one organic solvent.
 10. The invisible ink composition of claim 9 wherein said ink composition comprises about 0.0001-0.02% by weight said phthalocyanine fluorophore and about 0.05-5% by weight said ultraviolet fluorophore.
 11. The invisible ink composition of claim 9 wherein said ultraviolet fluorophore is selected from the group consisting of at least one ultraviolet absorbing stilbene, pyrazoline, coumarin, carbostyril, and pyrene compound.
 12. The invisible ink composition of claim 9 wherein said phthalocyanine fluorophore absorbs light within a wavelength range of about 650-715 nm and emits light within a wavelength range of about 670-720 nm, with said ultraviolet fluorophore absorbing light within a wavelength range of about 250-380 nm and emitting light within a wavelength range of about 400-650 nm.
 13. An ink delivery system comprising:a housing comprising at least one ink retaining chamber therein, said ink retaining chamber comprising a supply of an invisible ink composition therein, said ink composition comprising an invisible dye comprised of a phthalocyanine fluorophore selected from the group consisting of chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereof, said ink composition further comprising an ink vehicle comprised of water and at least one organic solvent; and a printhead operatively connected to said ink retaining chamber in said housing, said printhead comprising at least one ink ejector for expelling said invisible ink composition on-demand from said ink retaining chamber.
 14. An invisible ink composition comprising a phthalocyanine fluorophore selected from the group consisting of chloroaluminum (III) phthalocyanine tetrasulfonic acid and salts thereof, an ultraviolet fluorophore comprised of benzenesulfonic acid-2,2'-(1,2-ethenediyl)bis[5-[4-bis(2-hydroxyethyl)amino]-6-[(4-sulfophenyl)amino]-1,3,5-triazin-2yl]amino-tetrasodium salt, 1,5-pentanediol, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 1,3-propanediol, at least one surfactant, and water. 